Probably not earlier than Garcia did. He's more accurate but more tentative in the early rounds and not as explosive. I could see Khan taking a beating by the mid-late rounds and having the fight stopped. I thought Khan's style at its core could give Floyd problems, but I also thought he was much too green. He'll have to improve a lot to beat Floyd.
Earlier than Garcia? Hell he would have to jump on Khan from the opening bell to achieve that. If he let the fight come to him, by round 4. Round 8 at the latest.