Deb hit the nail on the head. To give you some hard numbers to go by, the general rule of thumb is that a weight loss of 10% is normal when a ball python fasts. With that said, I have had animals that go off feed for up to 9 months, and have lost as much as 20% of their body weight. Hoever, even though I hae given you some numbers, it still is a matter of health. If the snakes skin doesn't start sloughing or the spine doesn't become overly pronounced, adn the snake is still alert and active, it is probably just fine.
I hope that helps,