How often do you change your furnace filter?

[sommap]

Apparently I did not do it often enough.  Two different evenings, it shut down.  I got it going again by switching it off and then on again.  Since I had not had it serviced in years, and because I was worried about it happening if I left for days, a furnace man came out.  The problem was a dirty filter.  He did give it a tune-up though, which was needed.  Now they ae coming out once more this winter to check things out and then in the spring to service my air conditioner.  They are both a little over 20 years old and probably should be replaced with something more efficient.  I may do that this next summer.