If you know anyone that is an electronics hobbyist or ham, they'll probably have an oscilloscope, a counter or both. That and a couple of photosensors or hall sensors would tell you elapsed time between two points quite accurately.
You could get an estimate with a hex nut, a ruler, an assumption and a calculator. The assumption is that the piston accelerates linearly throughout its travel, probably not a bad assumption in most cases.
Orient the cylinder vertically. Put the nut on the piston. Actuate the piston, note how high it shoots the nut assuming that it doesn't hit any sheetrock, skylights light bulbs or pigeons.
The final velocity of the piston in ft/sec is the square root of
2*height*32.2, where height is in feet. Average velocity is half of final velocity, elapsed time is stroke (in feet) divided by avg velocity.If it moves 4" in 50 mS, avg velocity is 6.667 ft/sec, vmax is twice that (if the assumption of linear acceleration is correct), the nut will reach a height of 33.2 inches or 2.76 feet. OK the ceiling is safe,load one (1) nut and fire for effect. Ready on the left....
If it shoots the nut up 20 inches or 1.667 feet, vmax is sqrt(2*1.667*32.2) or 10.36 ft/sec, vavg is 5.18 ft/sec, stroke is
0.333 feet (4 inches) so elapsed time was .333/10.36 or .064 second.