How much line is needed for mainsheet?

Personally, I'd add another six inches to what the manual says. It lets you tie both ends of the halyard together through the cleat when the sail is off. That way you won't lose the ends if you're trying to raise the sail in a blow.
Richard, what does the halyard have to do with the mainsheet?

Anyway, how about a length for those of us using only a single purchase. I've sailed now in 18knot winds and you can still adjust the main fairly easily.