Rotating tires based on time or miles?

In a word: miles.

Time has an effect on the tires, but it will deteriorate the rubber all the way around at a given rate. A tire is good for many years (some longer, some shorter). It's the effect of driving on the tire which causes the issues which dictates rotating them.

The front tires of most vehicles will wear faster than the rear tires (noted exception: rear wheel drive performance cars doing burnouts :o). This is mainly due to the front tires having to turn, then along with the camber/caster/toe-in settings during the turning process, the front tires tend to wear much quicker. The rear tires just get pulled along. Even in rear wheel drive situations, the rear tires usually just pull in a straight line, so wear is minimized. To ensure the tires last as long as they can, you rotate the tires so all the tires will wear somewhat evenly, giving them the longest chance at survival (that and your pocket book).

Also, if you are worried about the maintenance schedule and causing the warranty to go bad on your tires, remember the tires are covered under the tire manufacturer's warranty, not the vehicle. The tire manufacturer is only going to be looking at the tread depth if there's an issue with them, not at if the proper rotations have been done on time. You want the tires to last as long as you can with them. Doing the rotations at equal mileage intervals is going to give you the best longevity you are going to get out of them.

One rotates on time or distance as some use their cars more or less and the tyres get "set" to the rotation.

But do make sure your car is not fitted with uni-directional tyres in which case they swap front / rear on same side. This is most often the case with snow / winter or wet weather type tyres as the tread pattern is designed to eject the water.

My winter tyres only do two seasons so I don't bother to rotate as they are only 50% worn but the uv degrades the performance massively.

In two words: It depends

First a disclosure: I am not an expert.

I agree with Paulster2's answer that, in general, you should go by miles.

But I'll add a critical factor, which I believe is important to mention: sun exposure.

Sunlight degrades tyre rubber, and can cause it to dry out and crack. This is especially dangerous for the sidewalls, as you don't want a sidewall blow-out. If a rarely driven vehicle is always parked in the same location and direction, chances are certain tyres get much more sun exposure than others. If this is the case, you may want to rotate your tyres after a set time period, regardless of mileage.

There is a way, of course, to prevent this need: park in a different direction (and location, if possible) whenever you drive your vehicle. For example, I have a vehicle that I rarely use, so once or twice a month, I drive it for 15-20 minutes (to warm it up) and then park it in the other direction.

By the way, if anyone disagrees with me, please do post a comment (or you can even post if you agree!), because if I'm in error, I would like to change my practices.