6 weeks. In 3 weeks, one flea can pass through its entire life cycle and can lay 1000 eggs. In another 3 weeks, those eggs can hatch and lay 1,000 eggs each, so within 6 weeks, it is possible to have a million new fleas.f the temperature is 95°F and there is 70% humidity, half of all flea eggs hatch within 36 hours. If it is only 55°F, half of all flea eggs will hatch within 6 days.