I found this on a leisure battery website under, 'battery care':
Never totally discharge a battery. It is unlikely that you will be able to recharge it again. In practice it is better to never let your battery discharge to less than 80 per cent of its capacity. If you allow the battery to discharge beyond half its capacity it is unlikely that you will fully recharge it again.
12.7 volts indicates a fully charged battery, 12.3 volts it’s half discharged and anything less than 11.8 volts the battery has fully discharged.