Just 5 minutes ago I thought about my cap and decided to check it. Before July I never had internet with a cap. Now with Comcast, I have 250GB. It was that or AT&T DSL/U-verse which also has a 250GB cap.
Of course I'm only a couple of days into the billing cycle but last month I used it a full month and I only used 45 GB. A while back, though I didn't have a cap, I used Netlimiter 2 Monitor and tracked my usage for several months. I averaged around 80GB/month down, and that was with a ton of streaming video on Youtube and such and I was unemployed at the time.