REM periods vary from person to person. You will just have to experiemnt with it and find your own REM periods. I start my REM sleep after only 2 hours of sleep. For others it can take up to 6 hours. I would suggest setting an alarm in 2 hour increments throughout the night for a few nights. Each time you awake, see if you have dreamed. If you have dreamed already, then you have slept a sufficient amount of time for using a WBTB technique. Good Luck!