I don't think it will take longer than 3 to 5 episodes, if even that long. Don't forget, there are people who reallllly know Stefan in Mystic Falls: brother Damon, ex Katherine, ex Elena and best friend Caroline. Although Caroline and Elena will be off at Whitmore College, it will not take long for everyone to see that "Stefan" (Silas) is not Stefan. Damon will be the first to see it, if not Katherine, or both. And since Damon knows where Stefan was planning on dumping Silas in the quarry, it likely won't take long for them to find him once they realize he is missing.