MSR claims 1oz/28g of fuel will boil 2 liters of water for the PR2 although they don't give an altitude for that figure. In theory you will use less fuel to boil at altitude since the boiling point is lower but you may use more fuel if you need to "cook" (ie simmer) since the lower boiling temp means things take longer to cook.
But based on MSR's number you can boil 2(227/28) ~ 16 liters of water with 227g of MSR fuel. If each drink or freeze dried meal takes 333ml (11floz) average of water that is (16000/333) ~ 48 drinks/meals. If you have, say, 4 a day then that's theoretically (48/4) ~ 12 days from an MSR 227g can using the PR2.
The stove you use, how you use it and how windy and cold it is where you use the stove will change the numbers.
Note: The brand/fuel mix you use may make some difference by I suspect not that much based on the testing I have found...at least until you get down to around freezing where some mixes will work and other won't without some warming of the can. A trick is to warm the canister with your body (eg keep it in your sleeping bag) to get it going and then place the canister in a dish of liquid (warm not hot) water while running the stove. Pure Butane boils at ~ 0C/32F and the best IsoButane/Propane mixes can apparently get you down to ~ -7C/20F. You have to move to an inverted canister stove to go lower (apparently ~ -18C/0F) and move to a different type of stove to go lower than that (eg: white gas) .