Depending on water temperature (so you’re not burning energy to maintain body temp), salt vs fresh (you’re more buoyant in saltier water), and body fat composition (fat adds to your natural buoyancy), I can definitely see someone who’s in decent shape managing 10 hours. I think the record is over 100 hours, so 40 is definitely going to be well above “normal” but not necessarily superhuman.
Depending on water temperature (so you’re not burning energy to maintain body temp), salt vs fresh (you’re more buoyant in saltier water), and body fat composition (fat adds to your natural buoyancy), I can definitely see someone who’s in decent shape managing 10 hours. I think the record is over 100 hours, so 40 is definitely going to be well above “normal” but not necessarily superhuman.