It hasn't crossed my mind to turn DS2 FF yet. With our first, it was really had to hold out until 21 months. He complained a lot and it was hard to get him toys, snacks, books, etc on long trips. But since DS1 is FF he can pass stuff to DS2 who is RF and DS2 hasn't really complained about RF yet. So I imagine he will RF for a lot longer than DS1 did.

Will you RF your second (or third) child longer than your first?