You have to be PATIENT and not offer to often, the more you offer the more stress, the less likely they will do good for you.
For the how long question it a judgement call that need to be made case per case depends on the overall look of the animal whether it has fully absorb it's yolk etc
Here is what I do.
If started on rat I offer a live crawler rat right after the first shed and do so once a week (not every 4 days or 5 days unless the first meal was successful) I do this for 3 weeks. At that point if they still have not ate I offer live hopper mouse once a week. I usually do not consider assist feeding with a healthy hatchling before at least 6 to 8 weeks after their first shed.
What is important is security, and also temps make sure your temps are not too high, for example in a small 6 quarts tubs you do not want to have your hot spot at 90/92 because it leaves very small room for cooler temps and there is not much gradient in small tubs like this. 86/88 is a great temp for a hot spot in smaller tubs, if if the ambient temp is around 84/85 in your room do not offer a hot spot at all.
Here are a few things you can do to help, use aspen as bedding it allows for them to bury themselves offering extra security. Have a small 6 inches plastic flower pot saucer in the tub (some need more security then others), add loose crumble newspaper, NO HANDLING, no constant opening of their tubs and again patience and make sure they are well hydrated.