It's really based on the individual. I've treated patients with beautiful skin who came to me because they had a mild breakout with one or two pimples. I've also treated patients who had waited so long to see a dermatologist, scarring was inevitable.
I always recommend seeing a dermatologist when you have tried over-the-counter products and they have not worked for you, or if the acne is severe. Large, painful pimples could be a sign of cystic acne, and should be seen by a dermatologist immediately before scarring has occurred.