Here's a chart I found:
Batteries that are being charged will be higher - the voltages while under charge will not tell you anything, you have to let the battery sit for a while. For longest life, batteries should stay in the green zone. Occasional dips into the yellow are not harmful, but continual discharges to those levels will shorten battery life considerably.
This page gives a scenario for depth of discharge effect on a gel battery: https://www.energymatters.com.au/components/battery-voltage-discharge/
A Sonnenschein Solar Bloc 100 AH Gel Battery discharged to a depth of 70%, i.e. with only 30% or 30 AH (amp hours) remaining, will have a lifespan of around 1200 cycles, which is quite impressive. However, if it’s only discharged to 50%, the expected number of cycles skyrockets to around 1700! If a cycle is a day, that adds over 1.25 years to the life of the battery.