Depends on the ruleset being used.
NFHS: Clock stops when a runner goes OOB, and starts again on the next snap.
NCAA: Clock stops when a runner goes OOB, and starts again either on the Referee's signal (with more than 2 minutes left in the first or second half) or on the next snap (less than 2 minutes in the half).
NFL: I think this is the same as NCAA, but I'm not completely sure.
Canadian: No clue.
Note that this assumes the runner's forward progress was not stopped prior to going OOB. If the runner's forward progress is stopped prior to going OOB, the clock will continue to run (Unless, in FED and NCAA, the runner has reached the line-to-gain). Watch the covering official in this case, if you see him/her doing the winding motion, the runner's forward progress was stopped prior to going OOB.