Hello,

I got a bike about five or six months ago with hydraulic disc brakes. This is the first time I've ever had a bike with hydraulic disc brakes. I was wondering, what are the signs that bleeding the brakes is necessary?

Thanks.

For SRAM: Annually, or when they feel bad and or inconsistent. For reals: it's a really good idea to do annually.
For Shimano or Tektro: Officially annually, but really when they feel bad or inconsistent.
Magura: When they feel bad or inconsistant.

"Bad" is a mushier feel at the lever mostly. It can also be instructive to invert the bike and see how much the brakes die when used in this position: it's indicative of how much air is in the master cylinder. This isn't necessarily bad inofitself, but when the brakes go to nothing in this position it's probably time to bleed. Inconsistent mostly relates to worse brake performance under hard and/or constant use.

I'd go with a year if it's not a DH or Enduro bike being raced. You can check for air by putting the bike vertically on the rear wheel and pumping the levers. If there is appreciable air you'll know...levers will go to the bar after a few/several pumps. The caliper bore seals on both systems will degrade and turn the mineral oil/DOT fluid black after a while so once a year minimum.
I used to race shifter karts and we'd bleed brakes every time we finished a session.