I would estimate about 6 weeks. I didn't weigh my hatchlings at birth but at 2 weeks they're about 5 grams. Weight gain can vary widely from hatchling to hatchling.
Mine don't gain much the fist week or two and after that I look for 1-2 grams per week. I try to cut back how much I feed them about 25g and keep it to 1 gram a week but they're too cute to walk away from sometimes!