The answer to this question depends on the type of flea treatment or repellent you are using. Some oral treatments like pills require a single dose in the first two weeks of treatment, followed by another dose after a month. Once the treatment is over, your cat should be flea-free for months.
Anti-flea collars should be worn for at least 21 or 28 days to deliver their efficiency and, after that, the results may last for another few months.
On the other hand, topical treatments like shampoos and creams will take more time to deliver the results, so some of them should be used for at least a couple of weeks or months.