Anything from a couple minutes upward. I've seen 13 hours, I've heard of a couple of days.
Why restart? If TempDB is full, make the files larger. Interrupting everyone because you've got the files sized too small doesn't make sense.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability