Average life is One to Ten years, Some run longer with lots of tender loving care but with a reasonable level of care, (Regular cleaning, and watering with DISTILLED water, Do not discharge too far, and recharge promptly) 7-10 years seems to be about right.
What will shorten it: Improper charging,, Leaving them run down for months on end, or over charging for extended periods (IE: Magnetek 6300 line on 120 volts (the unit was designed for lower voltage) for like six month) failure to water or using tap instead of distilled water. Discharging way beyond 50 percent SOC (or higher for Marine/Deep cycle but you ask about six volt which are normally DEEP CYCLE) Physical abuse and high temps will not help them either.
How to extend the life: When discharged, recharge promptly with a good 3-stage charger.. Check fluid levels frequently and add DISTILLED WATER (nothing else) as needed, Some think a slight topping with a couple TBl. of Mineral oil helps too, I won't argue the point since I have no valid research to reference.
An occasional slight overcharge to "Stir things up" a bit. These seem to help.