That schedule seems untenable even in the fairly short run. If you really need to work 90-hour weeks, six days of fifteen hours (and just one day off in stead of two) feels much more like something you could keep up for at least a month, perhaps even more.
If nothing else, 18-hour workdays leave at the most five hours for sleep (and only one for everything else, like mealtimes and hygiene), so you'll be pretty damn bushed come Thursday and Friday. 15 hours of work would leave seven for sleep, and double your everything-else time to a luxurious two.
If nothing else, 18-hour workdays leave at the most five hours for sleep (and only one for everything else, like mealtimes and hygiene), so you'll be pretty damn bushed come Thursday and Friday. 15 hours of work would leave seven for sleep, and double your everything-else time to a luxurious two.