Mold can begin within 24 to 48 hours on wet organic material. The controlling factor is drying speed, so removing standing water and running dehumidification immediately matters far more than any product applied afterward.
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The 24-to-48-hour figure comes from the industry restoration standards and it is a threshold rather than a guarantee. What actually determines growth is whether porous organic material stays wet, and for how long. What to remove rather than dry. Carpet padding is a loss essentially every time. Wet drywall wicks water upward well above the visible line and should be cut back to dry material, typically a foot above the high water mark. Cardboard, particleboard furniture, and stored paper are losses. Insulation in contact with floodwater comes out. What can usually be saved with fast drying. Concrete and masonry. Solid wood framing, if it is dried within days and moisture content is verified rather than assumed. Non-porous contents. The drying process is the whole job. Standing water out immediately, air movers directing air across wet surfaces, and dehumidification running continuously until moisture readings return to normal, which is typically several days and not the day the floor looks dry. A surface that feels dry can still be releasing moisture from within. When to bring in remediation rather than handling it yourself. Visible growth over a large area, water that was contaminated including any sewage backup, growth inside a wall cavity, or anyone in the household with respiratory conditions or immune compromise. Sewage backup is a separate category from clean water and should not be handled as a cleanup project. On products: bleach on a porous surface kills what it touches and does not reach the material below, and applying it to damp drywall adds water to the problem. Removing wet material and drying what remains is what actually works. And the underlying point: mold after a flood is a symptom. Preventing the next flood is the durable fix.