Everyday I use my Lumia 620 for some calls, texts, Facebook, Twitter, use of Camera, web browsing, WiFi on all the time, and a little bit of game and I can easily get between 15-24 hours. I didn't activate the battery saver mode and I set my brightness to low. I reckon with battery saver mode on, I can stretch it a little further.
You will notice that during the first week of use, the battery seems to drain real quick but after a week, things start to pick up and it lasts longer. That's what I heard from some Windows Phone users and I find it to be true.
I have to say it's not great, but not too bad either. I held a test with my friends' Lumia 800, Lumia 900, Samsung Galaxy Note and iPhone 4S and with the same frequency of usage, I get back home with my phone left with 20% of battery still while my friends went back with dead phones.