I think 5, 5 minute rounds of punching, kicking, grappling, takedowns, submission attempts, etc. is pretty comparable to 12 rounds in boxing. But 12 rounds against Mayweather is different, because you spend so much time and energy trying to find him and hit him. It takes a lot of energy out of you to keep missing with punches.
Once Conor gets tired, I really think Floyd is going to hurt him with a body shot.