The clones can either be braindead or with a full suite of memories.
In the braindead state they last about 1-2 months in perfect condition as long as you IV feed them regularly. Muscle atrophy will eventually set in, but they can be refurbished with treatment.
With full memory, they can last as long as you can force them to eat and don't do anything that would kill them. Certain reinforcements can be made to their body to make death more difficult, but anything biological will always have limits.