It depends on how widespread bellydance is in your area. If your audience members never see any other bellydancers, then you'll be the best they've ever seen. :dance:
Depends on the dancer, depends on the public, depends on the type of dance. For a good student, I'd expect them to be able to dance with a student troupe to a learnt choreography to an audience of friends and family in 12-18 months.
If you mean as a soloist in the general public not until you have a half dozen choreographies off pat and can improvise as well. Most people won't reach that level before 5 years - many never.