The most widely accepted statistic is that a single course of Accutane treatment takes away 80% of all your acne. They don't say 100% because it is not always 100% for some people. The 20% that might still remain might still be visible, or might appear and disappear periodically.
Personally, after I finished my first course of Accutane, I had 90% of all my acne gone, but a little bit still left. Nothing major though. It's been about 9 months since I last took Accutane and by this day I still have a pimple here, and maybe a pimple there, but nothing major. They aren't as big anymore though as they used to be before. And also, if I exclude any dietary choices that aggravate acne (dairy is a huge offender) - then my face will be almost 100% clean.
Sometimes, the first few weeks after stopping Accutane, the acne might come back... but it should disappear again and look just like it was while you were on the treatment.