Fleas can live up to two weeks without a food source. However, if you have a pet with fleas, and you leave your home uninhabited for several weeks, you will have an explosion of fleas. Fleas that no longer are feeding on the pet will reproduce, and the fleas you come home to are a generation or more removed from the original. Although they don't have a food source, they will still reproduce. The fun result is that whoever walks through the worst infested areas--pets or people--will immediately pick up many hitchhiking fleas eager for a meal.
A vet once told me that fleas can hibernate for months--I don't know if that's true.