If maintained properly a sealed lead acid battery will last up to 5 years.
However just one extended discharge can render an SLA battery useless. Even a near deep discharge can greatly reduce the amount of charge a battery will ever be able to retain. An SLA should never be fully discharged, as the battery does not need to be periodically discharged in order to maintain its performance, in fact, it is best kept in a fully charged state at all times.
SLA batteries can also be damaged by overcharging. A charger should be removed as soon as the battery has reached its full charge. They should be stored in a cool place when not in use, and can be damaged if stored in a hot area for an extended period of time.