Well seeing how they can go over a year without eating (not saying that is typical but it happens), so long as you ball python is not losing too much weight it is not a problem. 4-6 month fasts are not unheard of.

Keep tabs on their weight and I would say (don't quote me on that I don't know for sure the precise level to start worrying) if your snake loses more than 25% of their total weight rapidly (like within a month), make a vet appointment just to rule out illness (as in take a culture and check for parasites). However if they are simply fasting or brumating then there is nothing to worry about.

This is actually the first year I have not yet had any fast and I am still waiting to see if it happens.