SSleep time always go to it's default status, any ideas?

Hello all:
I do not want my mac book pro to sleep at all,
I went to Battery, and I'm trying to disable tern screen to sleep after? i tern it to off, but it reverts to it's Default status which is 2 minutes, is this a bug or what? do i have to do extra thing to achieve this? there is no Apply or ok or something, so, what can i do?
thanks in advance