No-one can give you an idea on how long it will last as every 2-stroke is different. I've seen an Aprillia RS125 last 3,000 miles before it blew up, and a Honda NSR125 go onto over 40,000 miles before it showed signs of wear. Neither bike is more reliable, it's just that one was better looked after than the other.
First thing you should do, is this. Take the bike out, and get it upto temparature. Once it's warm, take it for a ride and give it a good thrashing going high in the revs. Don't be abusive, just ride fast so it gets nice and hot. After you have done that for a few mies, go straight back home and get the spark plug out to do a compression test. Have a look at the colour of the spark plug, as it gives you an indication of how it's running.
BLACK - It's burning oil. Could be a sign of worn piston rings. Not good, but the Bike will still run for many miles so no need to panic.
BROWN/COPPER - Perfect. Engine is running with a good air/fuel mixture.
WHITE - Engine is running lean, and getting too hot in the bores. It's a ticking time bomb!
After that, do a compression test. Look online or in the manual for the amount of P.S.I you are looking for, then put the compression tester on, and crank the engine until the pressure won't go any higher.
After the compression test, give it a good service. Do this even if the previous owner said he done it. It costs barely anything, and it means you have peace of mind and can get to know how your bike works. Change the plug (get spares), check/change the air filter, check sprockets etc and give it a good clean. Then enjoy your Bike for the summer!
Hope that helps!