How long can leftover pizza be stored in the freezer?
I don't know if you can keep quality the "same", but yes, it's possible to freeze.  Plain cheese actually freezes better than one with lots of toppings.

For the absolute best quality, you need to limit freezer burn, which means wrapping it in something that will prevent moisture from escaping into the container surrounding it.  This typically means tightly wrapping it in a water-tight material.

I'd recommend first chilling the pizza in the fridge.  I would specifically not freeze an amount that you think you'd be able/willing to eat over the next few days.  (willing because you might get sick of pizza if you have it multiple days in a row)

Once it's chilled down, you have to decide on portions -- if you're going to be the only one eating it, you might want to only wrap it up in bundles of two or three slices, depending on the size of the slices and how much you typically eat.  If you have a family, you may want to go with larger amounts.

For wrapping, I suggest stacking up slices of a similar size and toppings, and then either:


Wrap each stack tightly in freezer paper, then tape shut any seams.
Wrap each stack in plastic wrap, then in a layer of aluminum foil.  (plastic wrap alone is too thin; aluminum foil may react w/ the acidic tomato sauce)
Arrange a couple of stacks in a large zip-top freezer bag, and suck out the air before fully sealing.


And you should label them before they go into the freezer with what they are, and when you originally purchased it (so if you find one months later, you know that you need to either eat it soon or dispose of it).

For thawing/reheating ... if it's still tightly wrapped, you can get away with thawing it in the fridge first.  If it's not, you'll end up with condensation on the outside, so you might be better going straight from the freezer to the oven.  (or a microwave on low power, then the oven).

Exact time and temperature in the oven depends on the thickness of the crust, the amount and water content of the toppings, and if you're going from thawed or frozen.  As a general recommendation, I recommend starting with a moderately hot oven (350°F / 180°C) if thawed; a bit lower if frozen, and cook until it's warmed through.

Other things that you can do to improve the quality of the pizza:


Turn up the heat towards the end if you like your cheese melted / gooey
Once it's warmed up a bit, add pre-cooked toppings and an additional sprinkling of cheese on top.  
Put it under the broiler / grill (top heat only) to get the top extra browned.  (also useful when adding toppings)
Pre-heat a sheet pan or baking stone, and place the pizza on that to get a crisper crust.  (but don't pre-heat to extreme heat like you would if cooking it from raw dough)
Adjust the oven shelf depending on how you like you pizza and/or how brown it already is.  Lower to reduce the amount of extra browning on top and cook the crust more; higher for the opposite.