There is no concrete definition, as it mostly involves how the member in question is viewed by other members.
Generally, posting a fair amount and being coherent will get you past newbie stage fairly quickly. Not posting a newbie thread helps, also. I've been a newbie multiple times because I kept leaving and coming back when no one knew me. I only got past the newbie stage once, but once I did, look what happened.
“I’m a stupid fatty and I love to play with my Easy Bake oven.â€