This is a Q there's no accurate answer for, it depends on so many things. You cat, its breed, its condition, its age, the weather, the other local cats...
In very vague terms, in my experience with my own, there is chance they will come into season for about 6 months of the year, usually spring-summer. It may last a month, 2, 4, even 6. They usually call (IE are fertile) for a week to 10 days.
Unless you really want more cats, please consider having the cat 'done'.