Usualy get 7 hrs usage and 20 hrs standby on wifi. While on 3g i get 4,5 hrs usage..
7 hrs on wifi only is not a great result, but it's not so bad either (it depends on what you do).
4,5 hrs in 3G could be due to a weak signal strength in your area.
In the place where I work I can barely reach 5 hrs in 3G, while in the place where I live, with a strong signal strength, I can reach 7-8 hrs ....