John learns to swim......

Around two week's ago my nan and grandad went on holiday leaving us to look after their 25 year old tortoise John. On the Friday me and my mum went to feed him and he was fine, happy as anything. Then on Saturday my nan and grandad came back, and to their surprise they couldn't find him. On the Monday we all looked for him but couldn't find him, then on the Tuesday we looked again, I went over to the pond and to my horror there he was in the pond fully submerged in the water. Me and my nan went inside the house crying leaving my grandad to get him out. My first reaction was that he was dead, but to our surprise he was fine, my question is how long can a tortoise survive fully submerged in water? mine was under there for around 3 to 4 days and is now stable & eating properly. We have now made sure he can't climb up to the pond again. Thank you