For heavy use, I've been able to get about 5-6 hours on average. But I also have to turn off 4G and only use 3G.
For moderate use on 3G, I've been able to get 8-10 hours.
The Rez has decent battery life for a smart phone. Especially since it has a HD screen. And 4G just kills batteries. Battery tech is not ready for 4G at this point.
I know you said you can't afford the extended battery, but it's really worth it. With it, I can keep 4G on, auto screen brightness, and with heavy it will still last 20 hours. Amazon sells the extended battery for the Thunderbolt for only $4 and works in the Rezound too, and then you can find the battery cover for the extended battery for about $15 - $20.
The Rezound is a great phone, especially because of the HD screen, it makes me want to use it so much.
Or you could get the RAZR MAXX. Supposedly that has really good battery life. But it doesn't have an HD screen.