I dont know what the average is, but personally I have around 450 hours. 400 of which are with two organizations I've volunteered at for a while. I've heard that schools are looking for dedication, and not volunteering at a ton of different places for a few hours each.
I think it depends on the schools that you are looking to apply to. ASDOH for instance that you have a high number 200+ hours because they are community based.