Nope. You should not need to replace it very often unless you are working in very dusty conditions.

In the spring I use it to blow the sand off the lawn put there by the town sanding and plow trucks. This creates a lot of dust when I try to blow it in piles out in the street. I have a corner lot, so my frontage is over 400 feet. The filter gets dark on both sides. I'm sure this is partly because it has a 2-cycle engine. When I try to blow dust out of the filter, it looks the same, so it may not contain as much dust as it appears to have. The unit does have a pre-filter, which cleans up pretty well. After the spring clean up, I use it mainly to blow grass or leaves, which does not raise much dust. I have never worn out an engine because I keep the oil and filters clean.
Perhaps I am changing the main filter too often due to it getting dark from the engine itself. Thanks for your input. Charles Ranheim

Can't help with the alternate filter replacement but it sounds like you're saying you're getting excessive spit back through the carb, wetting and oiling down the filter from the inside.

We went through a number of these things and had to fight tooth and nail with out Stihl distributor getting the things fixed. If yours is still under warranty take it back and raise a stink, don't back down. The best I can determine from the ever changing stories we got from them is the intake port height was wrong causing the excessive spit back, this isn't such a big deal on foam filters but very bad on paper filters.

Anyway, fix was to replace the jug with corrected port height.

Nevada
My BR420c is over three years old. There is not a lot of oil on the air filter, but there is some darkening of the paper element. It takes many hours of use before it darkens I don't think it warrants replacing parts at this point in time. I can run it for at least a year without changing the air filter. Even then, it may not be necessary if it does not contain abrasive dust. I have an ECHO SRM2601 string trimmer that seems to emit a little oil on the inside of the air filter compartment. I belive all my 2 cycle engines have done this on a limited bases. I always assumed this was a characteristic of that type of engine. I also add a little more oil to the fuel mix than what it calls for. The 50:1 mix calls for 2.56 oz/gal. I use closer to 3.0 oz/gal. Thanks for the information. Charles Ranheim