These 6V Golf Cart batteries are the mainstay of our small-to-medium sized systems.
These are true deep cycle traction batteries - designed specifically to power electric vehicles. The lifespan of these batteries is directly related to how they are cared for. If the battery is generally never discharged below 50% and has had proper maintenance (equalization charges & cell watering), you can expect upwards of 5 years of service. A battery that is not cared for will have a shorter lifespan.
When shopping for a deep cycle battery, be careful. The name "deep cycle" is used differently by different manufacturers. Generally, marine and RV "deep cycle" batteries are closer related to starting batteries than true deep cycle motive power batteries. Those batteries will degrade in performance much faster than golf cart batteries, expect a lifespan of 1-2 years.