How often should you bleed you brakes?

I have TRP Hylix on my commuting bike. Since installed in 2014 I've only changed pads, no bleeding needed. Levers still firm and braking very good. Did need to bleed my Ultegra (785) road bike front brake when bought in 2018, I think due to the OEM quick connectors letting some air in, but no other issue since.

A fine answer from Duncanap there, I have Hope Tech3 E4s on my ultimate commuter, I had hoped that they were a bit fit and forget, but they seem to need some maintenance.  I should clean and silicon lube (very sparingly) the pistons more than I do.  

There was a bloke that came to service the bikes at work, he mentioned that I should probably change the fluid in the system, but shortly after that I snapped the frame, but I still got the bleed kit in preparation. Took 8 months to get a new frame, warranty, but mid worldwide bike boom, once built I did notice sponginess on the rear.  Time to start doing stuff.  Good videos from Hope.

Flushed the system, Dot fluid, fairly creamy, then bled a week later just in case.  Pistons needed a bit of effort.

The thing with my disks is they still work quite well even if they need a bit of work.  I will try and do the pistons every 3 months, or if changing pads or if things get a little scrapey, and change fluid at least once a year.  Bleeding if necessary.

I think Dot tends to attract water, don't know how it gets in there but it does.

Hopes bleed kit is a beauty, and I got a Park piston press, which helps.

I have had disc brake bikes since 2016, I do about 9000 km per year spread over 2-3 different bikes. I live in Switzerland - so big descents of 10 km averaging 8-9% with plenty of heavy braking, and I weigh 90kg. I do all my own servicing and have only had to bleed one set of brakes and that was because I had to undo the hoses to change headset bearings. My bikes tend to be ultegra, but I do have a tandem which has SRAM brakes (they do use a different type fluid to shimano - so will behave differently). 

Maybe I have been very lucky, but I don't think bicycle brakes reach high enough temperatures for long enough to significantly degrade the fluids used, and I have not seen any evidence of water ingress. Mineral oil as used by Shimano is much less hygroscopic (it does not absorb water) compared to standard DOT brake fluids used in cars or SRAM systems.

I would say you only need to bleed the brakes if your lever becomes "spongy" and you cannot get the feel you want, or if the lever comes back to the bar on heavy braking (normally pumping the lever a few times will move the pads enough to stop this happening). 

This is not the official advice, but I think you can probably run shimano brakes for at least 3 years/30 000 km without a fluid change unless you feel the performance is not what you want. Shimano instructions say to change the fluid when it becomes noticeably discoloured - but most road bike systems you cannot see the fluid. So I rely on how the levers feel in operation.

I understand SRAM recommend changing fluid every six months as they use DOT 5.1 fluid which does readily absorb water. So the two technologies are quite different. I don't know about campy, or magura etc. I suggest you check what kind of fluid they use and if it is a DOT (glycol) fluid then it will need more frequent changes than a mineral oil or silicon based fluid.

I hope that helps a little.... the golden rule is if you notice a change in lever feel during use, or are just not happy then check it out or get it checked if you are not confident.