There is no single answer or average timespan to a question like this as there are too many different factors and personality types involved.
It may depend on how they left it. If they ended because she fukked up, and he was a really good dude then she'll probably kick herself for a while longer. If she dumped him over some shyt like fukking another bytch or constantly treating her like shyt or abuse, then it's more resentful, so she'll probably be quicker to move onto the next one.
Females also fall out of love while still in relationships, so she may just dead a dude and it may come off as sudden when it's really not.
But like I said.. It's complicated question.
You probably need to let her go though... She ain't coming back, she's with me now
