Twins and Triplets

[SimFan1228]

I know how to increase the chance of multiple births (twins and triplets) by listening to the kids music station or watching the kids channnel on the TV. My question is, how long do I have to do these activities in game in order for multiple births to happen?

Also, if I want a family to have twins, but not neccesarily triplets, do I have to do these activities for a shorter amount of time? In other words is there a threshold that causes the births to increase from two to three babies?