How often should hydraulic disk brakes need bleeding?
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].



[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