A good rule of thumb is to not go lower than 50% discharged. This will increase overall battery life. That means 12.1 Volts (for a 12 Volt system). The measurement should be made after the batteries have not been charged or discharged for 30 minutes or so. This will give a fairly accurate reading.
Next, it's always nice to know how many amps your solar panels are producing. This requires a current/amp meter. You can use separate, dedicated meters, or buy a Multi-function Meter that has many functions. I think a Volt Meter, an Amp Meter and a Multi-function Meter make a good compliment of monitoring equipment.
That way you can see at a glance what your system Voltage is and how many Amps you are getting from your Solar Panels. And, if more information such as AmpHours used, AmpHours remaining, Watts produced, etc, is desired, you can see this information using the Multi-function meter.