Answer: Never.
I will offer food once/week (after first shed).
After aprox. one month of non-eating (after-first shed) I will then offer once per week. If the food item is not eaten in 15 minutes I will force feed a pre-killed prey item a little smaller than the widest part of the snakes head.
For long-term non-eaters (3+ months): Once/month I will leave the food item (small live mouse-with rodent blocks) in the enclosure overnight and see if it is taken---Otherwise I will go back to force feeding weekly for another month.
When I say 'force feeding'-just to be clear, I am not talking tubing (that is different). I am talking assist feeding and then gently applying continuous pressure with dull forceps to gently force the food down to a point that the snake has difficulty expelling it. Out of hundreds of hatchlings/year-I sometimes end up with dozens of non-eaters (they tend to be from the same clutches).
I have not lost a single ball python hatchling using this feeding method. I also buy non-eating ball python hatchlings at expos on a regular basis. My latest non-eating acquisition was a GHI for $20 last year. Took 4 months of force feeding to get him to eat. He is playing the 'catch up' game now.
Note: All my problem eaters are only offered live food until they are established.