The fastest we saw in So Cal 10U TB was low 50's. From 35', that gets REALLY hard for most girls to connect with. Generally, those pitchers will dominate a game if they can keep it in the strike zone. I heard that there were a couple of girls at PGF Nats throwing mid-50's, but I haven't seen it for myself.
Part of what determines "average" is going to vary depending on the level of play, so I changed my descriptions a bit. Some generalizations (IMO)...
Fast = 50+
Good = 45-ish
Decent = 40-ish
Slow = sub-40 (you will probably see a lot of this is rec ball)
My DD has played against one of these girls a couple of times in 10U and I got to see her again a couple of weeks ago. As a 9yo she was consistently hitting 52 on the ball coach and had a decent change up she is a very intimidating pitcher for young girls standing at 5'11" her dad is a very big man at 6'10". I did not get to see her pitch her 2nd year of 10U but according to numerous sources she was throwing 56 consistently. She is not currently throwing the ball well, I heard she is having hip problems and hasn't been pitching to often and was only hitting 47 on the gun as a 04 12U two weeks ago. As for the other girl I have never seen her pitch but one of my buddies DD plays on her team and he said she is also very intimidating girl, very big and a lefty. I hear it said fairly often that she is considered to be the best 04 pitcher in the country right now. They are currently on the same team Diamond Sports Hotshots Mixed Gold 04 out of the Houston area. There was a couple of months during their first year of 10U that they were on the same team before the lefty split and went play for Diamond Sports Hotshots Gold. Were she lead her team to win the USFA World Series in Panama City Beach, FL, The Triple Crown World Series in Oklahoma, and the PGF championship all in the same month.