Or you could view it through the lens of working extra for a period, then taking some time off. Guessing you could remain productive at 9 hours of work each day, you'd save up a full 7.5 day off every five days (every week) -- and a full week off in 5 weeks of "crunch". If you work an additional 1.5 hours the numbers aren't that different for 8 hour work days, but that assumes you can maintain efficiency working 9.5 hour days for a week, and also doesn't translate to a full 8 hour day off for a week of "crunch".
YMMV etc.