Bit of a guess here but i'd say about 3-4 missions in a shuttle pilot's career is about the max. You have to fly at least 2 missions as pilot and then you get promoted to Commander- which you'd probably command about 2 missions. If memeory serves me right i think Nasa's most experienced pilot of all time was John Young??- He flew the shuttle twice, went to the moon twice and went up on a Gemini mission( 5 flights in total)