If we're talking average weight, I'm gonna say around the 1-2 pound range. Think of all the inexperienced anglers and people who fish streams or ponds. I'm not saying they can't catch big fish, not at all, I'm just saying their average is probably 1-2 pounds, and there are a lot of them. There are lots of experienced anglers that catch a lot of 1-2 pound fish, even if they don't want to admit it.
I bet during the Spring that average rises to 2-3 pounds. I could be wrong though.
This is just my guess and I could probably be way off.
This is probably one of the hardest questions in the world to answer.