I feel that this is a legit question at this point of his career. Seriously, the age of 35 is right around the corner, and I don't see him fighting too much longer. Do I have proof of how long he will continue to fight? Nope. Did he say that he wanted to retire undefeated? Yes.
There are few opponents left for him that public have talked up, but haven't given him a big chance of losing:
Pacquiao: People that truly know the sport, will be generous enough to say that he has a puncher's chance of beating Floyd. For the rest of the world or casual fans, Pac still has a big chance of beating him. I will leave this one in the air, as it will remain there until the fight is signed.
Martinez: He may give Floyd a few hard rounds, but I've mostly read that he loses to Floyd.
Cotto: He gives Floyd a few good rounds, but loses a UD.
Alvarez: He's too green (as I've read)
Ortiz (again): Not a chance...
As for this list, it "appears" that Mayweather only has one challenging fight left in his career, and who says that he will fight the rest of these guys anyway. Most will say that he ducked certain fighters in the past, which I wont even attempt to argue about, because everyone will have a difference in opinions. The explanation of opponents have been gathered by various boxing articles, comments sections, forums, etc.