With proper winter treatment there's no reason why you can't get at least 3 good years out of them. Probably 4 but you might find it doesn't spin the starter as well in the fall of the 4th year.
Without proper winter treatment two years........ maybe 3 if your winter shutdown in a little shorter. But you'll find you need to charge it in the spring.
Any instances of complete discharges (parking lights on by accident or whatever) or boosing tired batteries from cars will seriously degrade the battery and shorten the life further.
Proper treatment is to use something like the Battery Tender or equivalent set up with a light timer so it charges for about 1 hour per day all winter. This is much kinder to the battery than waiting until Spring and then trying to zap it with a quick charge. DO NOT USE car fast chargers on your bike battery. The standard auto "trickle" chargers are the equivalent of a "fast" charger to our bike batteries. Even these should only be used in dire circumstances.
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