How long should a new replacement battery last using only os x

i have just bought a used mac a1260. It runs os x snow leopord and windows 7. I know windows takes more power but how long should i expect the battery to last normally in standard mac use. Lets say just browsing the net. I'm lucky if i get 2 hours even though the battery starts fully charged at 4hr 30mins.

It also came with a dodgy chinese generic charger that would not allow me to use the mouse when charging but i now have an apple charger 85w (still chinese) model A1172 which seems to work ok.