Flea collars typically start working within 24-48 hours of being put on your pet. However, it can take up to two weeks for the full effects of the flea collar to kick in. During that time, the flea collar is killing off any fleas and eggs that have set up shop on your pet and will help to keep fleas from coming back. Flea collars are proven to be effective against adult fleas but may not be as effective...