i don't know the answer, but i ve read on the forum some where that some species have been known to go over an hour and some turtles can go all winter while hibernating berried in the mud under water. i've seen sparky go several mins with his head under water while soaking him. it was quite scary.
I took a tortoise to the vet once to have a large nasal abscess lanced. The vet tried putting the cone over her nose to sedate her, and he had to give up. She wouldn't breathe the stuff in. They can hold their breath for a very, very long time.
Tortoises and turtles are really marvelous creatures. So much we don't know about them. They continue to surprise me time and time again. I've timed mine at 10 mins.