for me it depends on a wide range of things. like how long were they together for, how serious it was, and who broke it off.
if they were together for a good amount of time i assume it was a farily serious relationship so i would have given the guy enough time to sort himself out. however if he was the one that broke it off, depending on the situation they may have been well and truly over their gf so "the time to grieve" wouldnt be as long. i never had a set time for when id make a move it depends on the situation.
not sure if that made any sense because i tend to ramble