When they start to loose alot of weight. I have had some go off food for over 3 months but they lost very little weight so it was no big deal. They are all back on food now and gaining again.
Depends on how old the animals is, how much he weighs, it's general health, etc.
A sub adult or an adult that is healthy and has a good body weight can go months without food.
If this is the case you need to make sure that your husbandry is spot on, if it is keep monitoring your animal make sure is well hydrated and wait it out.