WBTB - Sleep intervals?

So I myself am pretty experienced at lucid dreaming, having been lucid for about 6 years, but my friend here wants to try it out. I suggested WBTB, so we were going to do that, the only problem being that he feels well rested after 6 hours of sleep due to normal working hours. So what is the shortest amount of time you could sleep before waking up, in order to still achieve lucidity? I've read that good times are 3, 4.5, and 6 hours, is that correct? I feel like 3 hours wouldn't be enough to interrupt REM.

Sorry for the wordy post, new here.

Really, from what i know, time depends on person. Try different ones and find ones that work the best. :zzz:
Personally, i had success with my WILD WBTBs after 3 to 9 hours of sleep.

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