It's a case by case basis there is no "at so many weeks the animal should be assisted"
Usually once the shed I will offer food same prey for 3 weeks, if that does not work I offer a different prey for the next 3 weeks.
If that fails I use the brown bag method or ad crumble newspaper in the tub.
All that is based on an animals that absorbed it's yolk, and is healthy, I also go by their look.
Can you tell us more on the setup, temps, how food is offered and what you have tried.
How many out of the clutch have not taken a meal?