Thanks for asking your question on Just Answer. The life cycle of a fly goes through several stages.
Starting off when the eggs are layed, it takes 1 day for the eggs to hatch. There are then 3 larval stages (called instars). The stages take a total of 4 days. They then become a pre-pupa for 4 days and then become a pupa for 10 days (maggot). From the time they become a fly, it takes 2 days for them to start laying eggs.
I hope this answers your question. Please let me know if you need more information.