It really depends too much on the type of pizza.
Pizza Hut, I've left out in the open for 2 months and it was still perfectly edible when reheated (well, it was friend who left it out, and he ate it. I guess him being drunk helped a lot
Still, it looked and smelled like it was one day old. Pizza Hut is plastic crap, yo. Tasty and fatty, but plastic crap all the same).
Home-made pizza...Well, once. Maybe twice, though, like most others, I'd just reheat parts at a time.
Frozen pizza not at all. You heat it, you eat it. Seriously.
Big chain pizza (Dominoes and whatever): taste like cardboard when new, will taste like greasy cardboard when reheated. Don't. Finish it in one go, or eat it cold, or feed it to the pigs. Better yet, feed it to the pigs when warm, then butcher the pigs and make good pizza with the bacon.
Good restaurant pizza...Two, maybe three times. Eh.
Great restaurant pizza: not at all. If you buy really great pizza and you have any left, you're clearly not doing it right.