Starter/Relievers and Rest Days

If you have a starter/reliever that you've given a start to, how many rest days does he need before he can be used in relief? In the case I'm thinking about, he is a 6/3 who went 6 innings in his start.
http://forum-365.strat-o-matic.com/archivesn/
http://forum-365.strat-o-matic.com/archivesn/viewtopic.php?f=16&t=30113