In my experience I have worked as short as 5 days and as long as 60 on a hitch.
Having said that, and now working a regular 28 day cycle, I actually think 3 weeks would be good.
My reasoning based on 28 days:
1. first couple of days are spent getting back into the groove and possibly recovering from jet lag etc.
2. I think peak efficiency (ROV technical abilities and safety observation wise) is reached from end of 1st week to end of 2nd week
3. 3rd week is the hump probably more a psychological effect than anything
4. 4th week more glances at the calendar and peoples minds start to drift off onto what’s planned for the off-time
Where I am working currently offshore most people (non ROV/Dive) are on a 14 day cycle (not to mention that even the laundry hand earns more than an ROV pilot).