This depends on the type of filter you buy. If you choose a filter that is less than $10, it will likely last you less than one month. Other filters may last up to a year. Thicker filters last longer. For example:
1 to 2-inch filters: replace every 1 to 3 months
3 to 4-inch filters: replace every 6 to 9 months
5 to 6-inch filters: replace every 9 to 12 months