Fleas, just like any other insects, have what is called a life cycle. This means that, unlike vertebrate species, these small creatures’ life is regulated by a specific timeline. Fleas usually represent a long-term problem not because they have a long life, but because, like many other invasive insects - think about roaches - for example, they have an amazing reproductive capacity.
A female flea can lay as many as 20 – 50 eggs in one single day. Imagine what it can do in a whole month. All this means that if you are dealing with an infestation, it might take some time before you manage to completely tackle the issue.
Fleas can live an average of 50 days. The environment conditions, such as temperature,  influence the insects’ lifespan. Also, adults can survive without sucking blood from the host for as long as three months, which means they are quite resistant.