My wife and I live several hours away from the rest of our family and they complain that we do not visit often enough and they never get to see our 20 month old son.  He does great on long (3-5 hour) car trips.  We have noticed though that too many of these trips and he begins to freak out when we take him to the car.  We currently try to keep at least 6 weeks in between trips.

Are there any sort of guidelines for-

    How many hours a month a toddler should be in a car?
    How many days with normal daily routines should be in between days with abnormal routines?