I usually get about 6 years from a battery
WET very time, NEVER on trickle charge, I use a 4 amp charger with 1 hr /day (when not in use for a week) via a $10 time switch
(I will write a program for microprocessor one day, to do it )
PLUS my bike still has a REAL starter as back up
(hurts when it kicks back but)
Batteries can die for varied reasons usually its plates sulfated or crap falls off plaes and shorts out the cell in wet batteries.
Gel and AGM have their own dramas WAY TOO much to go into here