No matter which kind of Lead-Acid battery, the normal "wet" type, AGM or Gel, you should never let a 12V battery go under 10.5V or a 6V battery under 5.2V.
At lower Voltages than that the acid starts to eat the lead away and the battery very quickly is gone.
For our bikes, "wet" are no good, they are constructed for short bursts of high Amperage, like a starter motor and doesn't handle "deep discharges" well at all. They have much thinner lead plates than AGM and Gel which are much better suited for our bikes.