Mould generally needs 24 to 48 hours of sustained moisture to begin developing, which makes the drying phase after any water damage the single most important factor in whether mould becomes a follow-up problem or not.
Full drying goes beyond what's visibly wet, carpet underlay, wall cavities behind skirting boards, and enclosed spaces like under cabinetry all hold moisture longer than surface materials and are common spots for mould to take hold unnoticed if drying is rushed or incomplete.
Professional-grade air movers and dehumidifiers, rather than a household fan, are usually necessary to dry a genuinely water-damaged space thoroughly enough to prevent mould, particularly for anything beyond a very minor, contained spill.
