This is something that is still debated, but scientists have done some interesting experiments testing food left on various contaminated surfaces for different amounts of time. However, some of the research has not been peer reviewed (checked by other experts), so it is hard to draw firm conclusions.
In general, the findings from these different experiments showed that bacteria start transferring onto food almost as soon as it hits the floor, and their numbers increase over time. If the food hits a smooth bacteria-coated surface, like wood or laminate, it can become unsafe to eat very quickly, but if it hits carpet, the bacteria transfer much more slowly.
