Optimum amount of time before waking up for WBTB?

I'd like to give WILD/WBTB another shot. My question is this; what is the best (or close to) amount of time to wake up after you initially go to sleep? I've been trying four hours but I wake up so unbelievably tired I just drop off instantly. Is that too little an amount or something? How much would be ideal? Thanks in advance. :P

When I used to pracctise MILD and WBTB I slept for 6 hours and just lay in bed and thought about lucid dreaming and motivated myself that way, and then I just fell asleep and sometimes I had lucid dreams. But now I just sleep 6 hours and then go up for 5 minutes going to the bathroom and back to bed and then I just fall asleep.
I do a WILD technique here, but that is not important.
What's important is that you wake yourself up close to your rem sleep and 6 hour just feels good enough for me anyway, and then you stay up until you feel that thoughts like "Oh I am soo tired I just want to go to sleep again" is gone, but you still feel that you can go to sleep again. It's kind of hard to explain, but that is what I think works best.