There is no other way to say it! Some area's with dust storms will last only 3 to 4 weeks. If you have High pollen in your area, same thing! If you live in northern Alaska with none of that, Several months!
Depends on where you live and the weather/air conditions, but like Dom said, when it's dirty, get it cleaned.
This is what I am doing, I bought a spare Injen filter and Injen Filter Recharge kit. I recently installed my custom grill, and took the opportunity to swap in the new filter. Clean, dry, recharge the old filter and store it until it's time to swap out again - this way I will always have a clean/recharged one on hand. I drove 12,000 miles on the old Injen filter, it was noticeably darker because of the dirt.