How long do flea collars last?

Flea collars are not a good idea, and can actually be quite toxic to the animal. There is no way you can tell how much of the chemical is going on the animal's skin (if any) and over how long a period. Thus, you can't really tell when they aren't working. Another problem with these collars are that the chemicals in the collar will kill adult fleas ONLY, and not the larvae. A proper product given by your veterinarian is the best solution for flea treatment.