I can gain anywhere from 2-5 lbs while on my period (or, really, like a few days before it starts). Then it all goes away after 3-5 days, but sometimes longer. I can also usually expect a "whoosh" as well of about 2-4 lbs. It used to be more, but as I get smaller my whooshes also shrink.
If you're worried about gaining weight during your period, just make sure to stick to your food and exercise plans and don't worry about the water weight. It'll come off eventually.