Despite their busy jumping schedules, fleas can be tough to spot – they measure in at 1/16 to 1/8-inches long. And that’s just the adult, well-fed (not to say chonky) fleas we’re talking about, not young specimens. But just because you don’t always see them, it doesn’t mean they are not there. With a lifespan of about 100 days, fleas like to settle in and stay on one dog. That means once it jumps on your dog, the flea will stay there until it dies.