http://www.sialis.org/raisingmealworms.htm don't know if you've seen this website on raising mealworms. It says "Egg laying starts 4-19 days (average 12) after emergence".
Looks like your mealworm growing experiment is going well. Sounds like some chickens are going to be getting mealworm treats pretty soon. Isn't it funny how white the beetles are when they hatch out.