Recommended time is from few minutes to an hour. But it varries greatly among people, it can even differ for same person. The goal is to wake up enough to wake up your logic center, and so you can stay aware during the falling asleep process. But not to wake up too much, when your body figures, you are getting up and sleep for that night is over.

For a WBTB you should also get out of the bed, put the light on and do something LDing related. Like read your old dreams, watch an inspirational video, do some mantras and such.

But these are not rules cast in stone, just guidelines. If what you do works for you, you should be doing it your way.