That's very weird: my laptops stay plugged and on 24/7 for months at a time, and the batteries still last around 2.5 hours. These are 4 year old Li-Ion batteries I'm talking about (they used to last 3 hours when they were new).
I've gotten reliable service out of my mobile phones, but laptop Li ion batteries continue to feel (for lack of a more consistent and rational/analytical approach, on my part) like black magic, to me.