If there was no WiFi at my house, I'd probably use all 5GB of my plan, but because I do have WiFi, I only use about 800MB. It's nice to not worry about it though.
One app I recommend is 3G Watchdog. You punch in your plan and your billing cycle, and it will tell you how much you're using, and if it's almost used up, you can let it cut you off with another app for the rest of the cycle. It doesn't do that unless you're at 99% though.