How long a motor goes before a rebuild depends on a few different things. Has it ever gotten overheated, what riding style and conditions it sees (wide open pulls up hills vs. easy trail rides), warmup, tuning, break in, etc. It also depends on the motor family. Some motor designs just hold up better than others. I have over 3000 miles on my M7 motor with a head and exhaust system installed and have never been inside of it. I think there are guys who have 5000+ on these motors. I think the mark for new pistons and rings on a M8 motor is 2500? M1000 really depends on the tuning factor and the operator. Some guys can get a couple thousand out of them and some only get a couple hundred.
Most of the time if the cylinders are in good shape, the first 1 or 2 rebuilds only require pistons, rings, wristpins, wristpin bearings, and top end gaskets. After that I would pull the crank and inspect/rebuild it along with a new top end.
I hate to say it but the 800 Polaris motors are not known for their durability. 1000 miles max on a set of stock pistons/cylinders. Cylinders might still be ok at that point, but the pistons will not be. Poor manufacturing tolerances and high side loads on the pistons cause the piston skirts and sometimes the cylinder skirts to break off. Often times this blends the pieces up in the bottom end and wrecks the whole motor. $4400.00 for a new one.