If you live in an area where fleas are prevalent year-round, then you should apply flea treatment to your dog year-round. This is especially important if your dog spends a lot of time outdoors.
If you live in an area where fleas are only present during certain times of the year, then you can apply flea treatment seasonally. You should start applying flea treatment a few weeks before the flea season starts and continue applying it until a few weeks after the flea season has ended.
The type of flea treatment you use will also determine how often you need to apply it. Some flea treatments, such as those that contain fipronil, can last for up to 12 weeks. Other flea treatments, such as those that contain pyrethrins, may only last for a few weeks.
For a more definite answer: consult your vet
Talk to your veterinarian about the best flea treatment for your dog and how often you should apply it.