In reply to Suzanneinessex:
You could be, but unlikely. Everyone will only get so many chances before the assessor decides to defer people. You can probably bank on at least a couple hours and 3 or 4 lead legs a piece at a guess. You might be out 5 or 6 or more? Though above that I'm guessing is unlikely as well. In terms of times to bed it can depend on time of year as well.
I did my assessment in October, going dark at 5 or so, out from about 7-10/11. That was with two of us (the others dropped out before the exped). I had a mate do it in near mid summer, they were out until 4 or so in the morning to get the dark.
You may not even do night nav if the weather in the day is so bad, but I hear that's rare.
Again it all falls to the assessor. Others may have different experiences? Or if you have an idea of who your doing your assessment with you could ask for advice off people who have been with them (and not just about night nav too! Ropework is an assessor nightmare too, with them all having different opinions on what should be included etc).
Hope you and your chums have fun this weekend
Craig.