currently for FIS at all levels, 50mm total height from snow to bottom of boot sole.
flat skis are not all the same thickness, so with ski, plate, and "tuning shims" under heel and toe, you have 50mm to work with.
if you are participating in races where the FIS rules are being used for guidance, 50mm is your target, if not run what you brung.
good comments on difference between piston plates vs. vist, however there are many vist plates, not just one. and there are thousands of racers around the world skiing on vist built plates on many of the manufacturers pre mounted skis. if your ski already has a plate pre mounted by the manufacturer, i would use it. that's how the ski was designed and built. this applies to virtually all race skis being made today, with the exception of volkl, that builds and tests all of their race skis for and with piston plates.
of all the things that need to be figured out before you go out and race well, which plate you have on your ski, should only be taken into account if you have no life, no job, and a mind that will not let you relax and enjoy yourself at any waking moment