What factors determine how long a VRLA battery lasts?

In order of impact: temperature (roughly 50% life reduction per 10 °C sustained above 20 °C), depth of discharge (the deeper the cycle, the shorter the life), sulphation from deep discharges and poor charging, dryout from high temperature and over-charging, charge regime including whether the charger is temperature-compensated, and self-discharge during inactive periods exceeding six months.