Battery life is effected by what you use the laptop for.
You may get 4 hours if you are only working on documents.
You may only get 2 hours if you are playing games.
I think a good battery life is roughly 4 or 5 hours, but depending on what you use, 3 is fine.
It also depends on the battery cell size. With an Asus, I am assuming 3 hours is good.