Only the lap you're on. As long as you hit rewind rewind far enough (usually only two rewinds are necessary, but sometimes more), and you don't do anything else to dirty your lap after you're done rewinding, you can start the next lap clean. You can even do this if you dirty your lap at the beginning of a lap. I got a dirty lap on the second corner of Prague full reverse last night. I rewound the maximum amount of times the game would allow me (six or seven rewinds), and I was able to restart that same lap clean.