How often should I change my air filter?

I had a lose furnace vent cap on my roof. Piece of sheet metal that costs about $30. I don't do roofs as I'm in my 50s, have a bum knee and having shoulder surgery in two weeks. Called Axberg. Young technician came to door, looked up at vent and immediately said $450. I was floored. He stated it was $395 just to get on the roof. I then stated I had a minute to make up my mind or he was leaving and I would be charged $79 for the call-out. I knocked him down to $425. It took him 10 minutes to make the switch and 1 hour total with going to get the part. I called Axberg to complain the next day. Three days later they conceded the charge was a "bit" high and I was refunded $100. That still meant I was charged $325. There was never a $395 fee for getting on the roof so shame on that technician for gender discrimination, and Axberg you are charging about $295 / hr.