How long can a tortoise lay on its back?
until it's starving...
that depends on many factors and I don't think you can get a specific answer.
does it hot or cold rains or windy, did it recently ate,
how big is it
there's a theory that explains how the huge tortoise came to the Galapagos maybe they somehow drove to the sea and since they were so huge they survive the journey and made it to the Galapagos by sea