Gnats go through four stages in their life cycle. It starts with an egg, then larva, pupa, and lastly, adult.  Once the egg is laid, it takes 3 days to hatch. Once the larva is out, it spends 10 days feeding on roots before developing into a pupa. 4 days later, the adult gnat emerges. An adult gnat can live for up to two weeks. Most of the damage is done during the larva stage.