There are many stories to this. Some people say theirs leak all the time. Some never, so do when they get over, some have to be pump every two weeks or every ride.
If you get a new air fork, it will usually last at least two years unless you got a LEMON product. In that case, the seals might be bad and you might want to get it service either by yourself if you know how to or bring it to a local bike shop or professional fork rebuild places if you can afford to like PUSH.
With the two air fork that I have, one is a 99 RockShox SID SL Dual air. I used it for a season when I bought it back then. Lefted it until 05, used for one season of 05, and started to leak in 06. Thats because the seals and cartaiges stuff has been sitting so long as well as they got old.
My other one 04 Fox Float RLC used for 2.5 seasons now and NEVER leaked at PSI.