That is a difficult one as it depends upon the individual and his or her metabolism. Some people have bladders like a pea, so to speak, you know what I mean? I wouldn't worry so much about that, though, because getting up to use the loo before an induction won't affect your chances. I do it all the time, in fact. After having slept for about six hours, feel free to use the toilet because when you next go down, you will have nothing but plenty of REM—fertile ground for lucid dreaming.