House flies are ready to breed mere 2-3 days after their birth. Then, they proceed to lay about 150 eggs at a time in spoiled or leftover food, garbage, and organic waste. Over its short lifespan, a fly can lay up to 3 thousand eggs. The eggs are tiny and invisible to the naked eye, but they gestate rapidly. It takes a day for larvae to emerge. Commonly known as maggots, they will increase their size by up to 800 times in a week. The maggot pupates and in a week transforms into an adult fly. All stages of development from egg to an adult take 12-14 days. It crawls for the first 2-3 days of its life, waiting for its wings to grow stronger. At this point, the fly is ready to reproduce.
Their reproductive capabilities are alarming because in a few days there could be an army of insects causing a house flies infestation.