Lead acid batteries do not like being completely discharged. I have used the following chart for reference, in most cases i only discharge my battery to 50% DOD (depth of discharge).
Even deep cycle lead acids life (max charge cycles) will be extended if it is not completely discharged. It is also generally recommend to completely charge your battery after use – i.e. take the battery back to full charge in a single cycle.
My battery is a deep cycle 24v bank, so i generally only discharge (under load) to about 24.0v (see ATS post)