Give classes: When you have enough knowledge of the dance, in theory and practice, and can teach it properly. Over here many dancers are in such hurry to teach they only take classes for 1-2 years. I've never seen anything good come out of that. But it COULD work for someone really talented.

Perform: When it looks good and the dancer has such knowledge of the style, music and her (/his) body.

These factors don't just "show up" after a certain amount of time. It varies a lot from dancer to dancer. Mostly the advice from one's teacher is a good lead. (Even if one of mine was simply mean about us performing, she kept saying we'd "bore them"...)

That is such a personal opinion and I can only give you mine. I only apply this rather tight view to myself and not others.
To me, for teaching I prefer a teacher who has been dancing for many years and teaching for many years as well. My main teachers have been dancing BD for around 15 years. I know they have a solid background in many areas including technique, performance, teaching, choreography, and improv. And I'd like to add that dance experience does not a good teacher make. Being an effective teacher is a skill all unto itself. And I spent ungodly amounts of money on this hobby that I want the best of the best.
As far as performance- anytime baby! Where I am at, there are many venues and opportunities for dancers of all levels to perform. I think getting out there and performing something fun and lighthearted with the idea that the performers are students (takes a little pressure off that way) is a good way to validate the hours and $ spent practicing.

Of course there are always exceptions ;-)