Mold can begin to grow in the walls of your home within 24-48 hours after a flood. The walls of your home can work like highways for transferring the mold spores from one area to another. Even though you do not see the mold spores, it can be present. Mold colonies that are large enough to be seen can take 18-21 days to form, but the mold can still be present as early as 24 hours after a flood.