There is no number.

Sixty years is not enough and eleven hundred years would not be enough and neither of those sentences has one thing to do with a six-year-old who will not sleep with the door closed.

What the numbers actually bought me is that when my daughter is fifteen and asks, I can say a figure and a year and she can do arithmetic, and the answer will be that he is not coming out while she is a child.

That is what a sentence is for.