nah, in real live u would calculate... for example before the race u see it has 50 laps, than u tell ur crue, u wanna come into pits in lap 25 thats it.
and even if u come earlyer, u just tell it your crue for example when u get into the lap u wanna come into pit. than the crue would need maximal 15 secs to get ready.
even if u would call them when u re entering the pits, and your crew would be at the end of the pit, the would be ready when u come :S