According to IICRC, Institute of Inspection Cleaning and Restoration Certification, frequently used carpet should be professionally cleaned by a qualified technician at least once a year - perhaps a little longer interval for infrequently used carpet and even sooner for carpet in homes occupied by respiratory sensitive or allergic persons or in homes with indoor pets.
In terms of mold and wet carpet, there is no defined time frame to predict when mold will grow in carpet. For any material to support common indoor mold growth, it must:
1. Be organic
2. Be damp or wet Have a moderate temperature (68ºF-86ºF)
3. Be in a stagnant air environment
4. Have these conditions present for several days
Most tufted carpet is made of plastic face yarns and backings, and synthetic latex. There is nothing organic to support mold in the carpet itself. Therefore, mold cannot technically grow on carpet. However, average household soil is about 40 percent organic (12 percent cellulose, 12 percent protein, 10 percent oils, 6 perfect food stuffs); therefore, mold can grow on the soil in carpet given the right conditions listed above. However, if carpet is kept clean and dry, there is little or no possibility for mold growth. Even with soiling and normal household conditions found in most homes, it takes considerable time for mold to grow on carpet soil, and then only if dampness is present.