As far as a WBTB period, about 60 minutes is supposed to be the 'ideal' time but this absolutely varies by person, so you should experiment around. If 60 minutes makes you too awake, and gives you too much difficulty falling asleep, then reduce it a bit. If not, maybe extend it. Whatever feels right and works for you is right 180 minutes sounds like a long time, but if it can work for you then go for it, especially if that's the time period you've got to work with.