rotating quarterbacks

Can you be a bit clearer?

I think you mean can the starting QB come back in after they have been benched or replaced. The answer is yes but only in the 4th quarter (post-1995 ruling). However it is rarely done. This is true also of the 2nd string QB.

Oops, I lied
http://www.ehow.com/info_12118524_rule-quarterback-reentering-game.html


{Edit. I stand corrected. There was a new ruling about replacing QB since 2011:
Rule Change in 2011
"In 2011, the rosters were expanded to 46 players, which meant the third-string quarterback was a full member of the active roster. Because of this, the third-string quarterback is allowed to come into the game for a short period of time during any point in the game and then the first- or second-string quarterback can immediately re-enter, instead of having to wait until the third or fourth quarter. This rule allows for the first or second quarterback to get a rest or mentally prepare to re-enter the game if he has become frazzled."}