How often should I change road bike tyres?

How long a tire lasts depends on a number of factors, including what type of tires you ride, how much you weigh, the conditions you ride in, front vs rear tire, etc. In general, a good set of tires will last a couple thousand miles.

When the tire is totally worn out, you can usually see threads beneath the rubber in places. Alternatively, the tire may start to bulge in spots. In practice, road tires rarely get to this point. Much more commonly, they start accumulating a lot of cuts from glass and other debris in the road. When you start getting disproportionately many flats, it's a good idea to replace the tire (you can use the tire longer, but you'll probably waste more than a new tire's cost in tubes).

Change them when they're worn out -- either cracking on the sidewalls (due to age, sun, and underinflation, possibly exacerbated by poor tire quality) or the rubber in the center is getting paper-thin and you start getting a lot more punctures. Simple lack of tread is no problem.

(And it should be noted that in the OP states his tires are "bald" in 500 miles -- a very short distance. Unless he'd been like a kid laying rubber with the brakes, the tires in question likely have a center strip that is essentially tread-free to start with. This is a common design to minimize rolling resistance.)

Always wipe off and inspect the tires after every ride. Look at the sidewalls as well as the tread.

I get about 2,500 miles from Continental Grand Prix 4000S, a high-end road tire. The rear tire always goes first. Then I rotate the front tire to the rear, and put a new tire on the front. I've never had to replace a front tire due to tread wear.

Some Continental tires also have wear indicators -- two little dots in the tire -- to let you know when the tread is gone. That's where I get the 2,500-mile figure from; it takes me about that long to get to the point where the wear indicators are shot.

Schwalbe give the following figures for their tyres:

As a general guide, you can expect a tire mileage of 2000 to 5000 km from Schwalbe standard tires.

The tires of the Marathon family usually last between 6000 and 12000 km. With the light Marathon Racer and Marathon Supreme, the performance is a little lower (approx. 5000 to 9000 km). The Marathon Plus is outstanding with its extremely high mileage of approx. 8000 to 15000 km.

No useful mileage data is possible for MTB tires because the influence of riding style is too dominant.

Our racing bike tires Durano and Ultremo last from 3000 to 7000 km.

That page also has some pictures showing tyre wear

I usually get 4000 miles from a Continental 4000S. Years ago I got 5000 miles from a Performance brand tire. However, having neglected to keep an eye on it, the casing completely disintegrated in the middle of a cold, December ride. So keep looking for wear.

I commute about 200 km a week. My last set of tyres - Specialized armadillos - lasted exactly one year. There is still at least 6 months rubber left but the tread rubber has started to delaminate from the canvas. So they have lasted me 10,000 kms which the bike shop reckons I should be happy with. I have managed more than twice that time (and distance) on a set of Michelin World tours. Cheaper tyres (1/3 of the cost) normally only last me 2 months before they are completely worn out. They are a false economy.

Sheldon talks about replacing tyres (or "tires" for Americans)

Many cyclists waste money replacing perfectly functional tyres simply because they're old, or may have discolored sidewalls. If you just want new tyres because the old ones look grotty, it's your money, but if you are mainly concerned with safety/function, there are only two reasons for replacing old tyres:

1. When the tread is worn so thin that you start getting a lot of flats from small pieces of glass and the like, or the fabric shows through the rubber.
2. When the tyre's fabric has been damaged, so that the tyre has a lumpy, irregular appearance somewhere, or the tube bulges through the tyre.

Cracks in the tread are harmless. Small punctures in the tyre such as are typically caused by nails, tacks, thorns or glass slivers are also harmless to the tyre, since the tyre doesn't need to be air-tight.

There's yet another reason to replace tyres, not covered above:

When the bead snaps and pierces the sidewall.

An unusual failure mode for a Schwalbe Marathon Plus.

Got through 2 Continental Sport and three other tyres in about 4 years, which is about 20,000 km. Three had a hole in the sidewall, one had disintegrating rubber with viable threading, and one actually exploded!

Fronts seldom wear out in the life of a bicycle, per se, but they do age, harden and eventually delaminate. This said, rotate-on-replacement of the rear, so that your new tire is your steering tire, and your aging tire is your drive tire (that will be consumed). Schwalbe is right on for tire-mileage, on premiums - but a lot of your 'no-name cheap imports' have tire life measured in hundreds of miles, not thousands, so be aware, that Schwalbe-Michelin-Continental-Panaracer price is justified! Decent mid-range name brand tire is good for 1,500-2,500 miles, road miles, with things like road quality, weather (wet roads wear a tire more, actually, due to slippage), biker weight, and riding style (no MTBs can realistically be estimated). On a 'desert note' (where the thorns called 'goatheads' roam freely, also be aware that while you can add rhinoliners, tube sealant, thorn tubes, kevlar tires, eventually you are going to 'get-got'. While a small thorn is not life-ending to a tire, to prevent later fiber separation within the tire's belt, it is VERY wise to find the hole and use either rubber cement or super glue to 'fill' the thorn hole - to hold the hole closed. It is NOT for sealing air, but to help restore some integrity to the tire's structure after a thorn pierces it...rider's decision here, but I've never had a 'fiber separation failure', while I've had tubes that come out at tire-change, that literally had HUNDREDS of little green dots on them! (where Slime had filled a 'goathead' thorn hole location).

I've put my crossfit bike through the ring a couple times. My back tire has about 15,000 miles on it while my front has almost 35,000 miles on it. It's been through floods, ice, dirt, and just about any kind of weather you can think of. The wear marks are still only at the half life point. As long you you take care of your bike and keep the proper air pressure when you ride your bike it will last a very long time.

25mm Surfas Seca lasted 1140 miles. 25mm Thickslick lasted approximately the same. 190 lbs, daily ride 27 miles, 1,500 vertical.

How do I know if my tyres need to be changed, and how long should I expect tyres to last?

It's impossible to give any other answer than "rear tires wear faster than front tires" and "change when you see the cords". The wear rate depends on the quality of the tire, its tread thickness, your weight, your riding style, and many other things apart from the number of miles.

However, to change tires, move the front to the rear and put the new tire to the front. A front tire failure is far more dangerous and thus it's better to have a new front tire than a new rear tire. Furthermore, if you always change the rear tire only, the front tire would last practically forever and would never be changed. This would mean the front tire is used so long that it is at a risk of degrading due to its age and the integrity of its casing rather than treadwear.

The proper time to change a road bike tire is when you see the cords under the tread. Unlike cars that need resistance against hydroplaning (and thus a minimum tread thickness), with bicycles whose tires cannot hydroplane you can safely wear a tire to its cords but no further. Ignore any treadwear indicator if your tire has such indicators -- the manufacturers err on the side of caution, meaning you get too little miles out of your tires.