Seeing as Mercutio has not been on ZU for three years, I'll answer that one for him.

A decent amount of time, in my opinion, is around three weeksish and more.
I mean, someone has gone out of their way to create a sig and avy for for you, the least you can do now after demanding images ffrom them is to display them and respect their efforts for a while.
You're better off asking the sig creator themselves though, what a decent time is, as no doubtedly, each one will have a different opinion.