This is something very individual, and that’s probably also why you have got several different answers.
However, people tend to have similar REM cycles that occur in approximately 90-minute intervals, and one of the most ideal ones to aim for takes place roughly 6 hours after you fall asleep - you can also aim for 7,5 hours of course, but that might become too much sleep if you want to attempt a WBTB.
The reason you should wake up during REM cycles is because that’s when you dream, and it’s much easier to remember dreams if you wake up straight from them.
Yeah, that’s very normal.
Actually there are several proficient members on this site who have mentioned these kinds of periods, when they had no success for as much as 20-50 days and then got lucid dreams again.
You should take a break if you feel exhausted or if you take it too seriously - remember that lucid dreaming is supposed to be fun and exciting, it should never feel like a burden.