Honestly it depends a lot on your riding style (how hard you've ridden it), how well the rings seated when it was broken in, and how often it gets ridden.
There are still 01's out there with original pistons and rings. Not too long ago I had a 1981 honda atc 185 that was all original, piston, tires, everything.
The best way to check is to do a compression test. This is assuming you don't have any of the tell tale signs like obvious power loss, drastically dropping oil levels from blowby and the accompanying smoke out the exhaust, and a bike that no longer runs. With the oem cam decomp setup you should be shooting for low 130's (in psi). Without decomp it's around 180psi. If its significantly lower, time to replace. If you're looking at a trail bike you can probably keep the oem piston and just install a new set of rings after the cylinder gets a fresh hone.