How often should hydraulic disk brakes need bleeding?

I think the "regular bleeding" that every hydraulic brake needs should be at most once every Three to Five years, unless some problem happen.

Even with sane sealings, there is always some chemical degradation and contamination of the fluid, which decreases its performance, but it happens very slowly if at all, I think.

If the brake response gets mushy, also check the brake pads. Personal experience: I saw no need in bleeding the brakes before changing the brake fluid (on a DOT system). Maybe your system is not sufficiently sealed?

The need to exchange the fluid depends on the type that is used in your brakes. And this should be self-evident: Do not, under any circumstances, change the type of fluid used in your brake! It will most certainly degrade the gaskets.

DOT

If your brakes use DOT (classified by the US Department of Transportation) brake fluid, i.e. "DOT 5.1", the fluid should be exchanged every 1-2 years [1]. With time, the fluid absorbs water, affecting its compressibility. This is by design, since non-absorbed water would affect the compressibility even more.

Mineral Oil

Mineral oil, however, does not need to be changed so regularly. Unless you open the system, the oil stays good for up to 6-7 years [2].

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[1] Bosch Automotive Handbook, 7th Edition, ISBN 978-0-7680-1953-7
[2] http://www.magura.com/de/produkte/scheibenbremsen-2012/prod/mt2/info/faq.html

Typical hydraulic disc brakes use an 'open' design to allow for automatic pad adjustment. They have a small reservoir at near the brake lever for 'refilling' the extra brake fluid that is needed when the brake pad needs to be pushed a little closer to the disk in order to compensate the brake pad wear.

This type of design leads to less hassle in respect to pad adjustment, but has one main disatvantage: as more and more fluid gets pumped from the reservoir towards the brake cylinders, the oil gets replaced by air. Now if the bikes position changes from the regular upright position to e.g. upside down, air bubbles can slip to the brake hose and that causes the spongy feeling in the brake levers.

So you don't necessarily have to open up the system to get air in, that's quasi by design. With a little luck you can pump the air back to the reservoir, but normally you need to bleed the system.

As you use the brakes and the pads wear down, the oil will heat up and expand. This pushes oil back up into the reservoir, and when the oil cools off the pistions will suck away from the rotors.

Look at the brakes and you'll notice the pads probably aren't floating against the rotor as they should be, but when you squeeze the handle most of the motion is moving the pads to the rotor. When pads and rotors are new this doesn't happen because the pads are thick.

Bleeding doesn't help this, as it's normal as pads wear. The pumping you do during braking doesn't move enough fluid back into the lines to get the pads to set back next to the rotors.

The quick, easy and proper way to fix is to take the wheel off, and continuously quickly pump the handle until the pads meet in the middle of the caliper. When you can't squeeze any more then you've loaded the lines. Then using a clean flat tip driver, gently pry them open just slightly enough to get the rotor back in. Your brakes will feel like new and should feel good for awhile longer. The more your pads wear however, the more you will have to perform this little reset.