General pleated AC Filters are changed every 2 to 3 Months Depending on use. Environments with Smoke, heavy dust (ie Desert) or pets may need more frequent air filter changing.
Cheaper flat fiberglass filters may require changing every 30 days and may even be labeled as a 30 day filter.
Whole House AC Filters with a size from Approximately 3 to 6 inches in width can have a life spans as long as 6 to 12 months depending on environmental conditions.