In general, you should be able to tap to signal submission, and the hold should always then be released. Of course, you can agree to some other rules, but be sure you understand what they are. Some people are into sleeper holds. I am sometimes asked to recommend girls who will do sleepers. I will not make such recommendations.
Sleepers work by pressure on the carotid artery, which shuts off the blood supply to the brain.
There is too fine a line between unconsciousness and death here. We're not talking about some injury that you'll recover from. We're talking about possible brain damage and death. The first version of this paragraph implied that the brain could go as much as three minutes without oxygen. Some of the girls on this site probably aren't that good, while some are almost unbelievably strong, and some are terrific wrestlers.