Need to consider how many amps you are drawing when you decide how far to take them down to.
For instance - if you were drawing 2 amps and wanted to stop when they were 50% discharged, you would need to stop at around 12Volts. If indeed you kept going until 11V at this small current, the battery would be very close to flat.
Draw 50 amps (small inverter) and you should stop at (say) 11V - because there is higher internal voltage drop to take into account, plus other factors - and you will find that if you then drop that load off, the voltage will rise rapidly and if you have judged it right, will settle out at around 12V, so roughly 50 % discharged.
Take out 200 amps - as some do to drive a large inverter - and the voltage will immediately drop well below 11V and perhaps closer to 10 volts and for that load, you can stop at 10V to end up with a 50% discharged battery.
Note that I'm not saying you should drag 200 amps out of your battery because that too will reduce the lifespan of the battery, just as much as excessive discharge will.
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RVs. USA - Airstream Cutter; in Australia - MC8 40' DIY Coach conversion & OKA 4x4 MH; in Germany - Hobby Class C; in S America - F350 with 2500 10.6 Bigfoot camper