i have never noticed a difference in skirt length between manufacturers.
Of course they're going to give you a 20 hour max limit, life of the piston is dependent on factors such as rider skill (revving it out more) oil quality, and several others. That said, logically a person of average skill level is going to need less pistons than Justin Barcia, now when Vertex says that, they don't want any sort of liability (and more sales out of joe blow) for when a local pro who bangs a 250f off the rev limiter hops on it, he probably should be changing it every 20ish hours. Overkill for mortals like us though. 50-70 hours is a safe margin for those of us without super powers imo and expierence/word of mouth from the guys that actually turn the wrenches. Of course I run my oil for 10 hours too, but hey, scientifically, through oil tests and what not I could find 0 indication that it needed to be changed sooner. But thats just the way I do things.Read what Vertex has to say about the life span of a two and four stroke piston.
What is the life of a two-stroke and four-stroke piston?
Piston life is relative to the operating conditions of the engine, so no exact life expectancy can be given. In a perfect world, about 20hrs is the max usage you want to try to get out of a piston. Again, this is all relative to use, maintenance, and operating conditions.
If you had the budget, it would always be best to change the piston and rings sooner than that. I could never afford to it that often myself. So I use the oils that I think work best to help prolong any replacement cost.