depends on the quality of the leds,
also if you drive them with to much current they will be nice and bright for a few mins or hours but after that they go down hill.
leds don't like spike or surges or transients.if you want to get the maximum life out of them they are best run on a regulated dc power supply.( with a surge protector on the output of the power sup.)
in a piball machine the g.i is pretty constant (but its a.c.) but the voltage to the insert lights vary a bit which can make a big difference to the current getting to the leds
if the resistor that is in series with your led is worked out to drive the led at 25ma @ 5volts & your supply goes to 6 volts then you will be over driving them.
also they will run on a.c. but they will flicker slightly, i.e. if you are running them on a 50hz supply they will only be on 25 times a second.
don't forget they are a semiconductor.
if you need anymore info on this i will be happy to answer any questions as best i can.
i was once employed to design circuit boards and power supplies and assemble these for led lighting for 10 years.
hope this helps