A reasonably fit young man should be OK for at least 2 hours before a break, and should be able to continue 12 or more hours. I used to work just as hard when I was working on the building of the M1 motorway and that was 12 hour shifts.
When are you looking to do the first practice run ? The weather up heres starting to get foul, so perhaps a small practice run might be useful or we could be rowing proper!