I don't know if you still need the advice or not, but I can tell you that 3 people can definitely get that done in a day. The first round might take two full days, or 6-7 people in one day. But as long as the schedule remains weekly after that, then nothing should be so dirty to require extra attention. Provided the owner isn't trashing it every week, then regular weekly maintenance cleanings should be a breeze for 3 people in 6 hours.
My previous employer always ran us in teams of 3, and we would hit 4-5 boats in the 30-45 ft range each Friday. Anything over 50 ft usually got a half-day. If we ever had something 60ft or up, then we'd give it an entire day.