I can almost always tell when my HVAC filters need to be updated, it is usually about every 3-4 months in time.
Changing my HVAC filters has become an straight-forward task as I have done it multiple times in the past, while my hubby was not away on labor trips, I would end up finding myself having him do all of the service tasks around the house, but, when she was away, it was up to myself and others to learn how to do all of the service tasks.
I am usually able to tell that my HVAC filters needed to be changed by the duration of time the a/c equipment stays on while I was in the afternoon. If the method comes on for a little while & turns off for the same time, that usually means that the HVAC method is filtering well & officially cooling the house. If the HVAC method stays on for a long time & then comes back on shortly after that, that almost always means that something is not right, but however, this is not consistently the case; This is almost always true while I was in the summer time months when the uneven temperatures rise to higher than usual, and another sign that these HVAC filters need to be fixed up is if I start to notice my youngsters sneezing more than usual. This is not consistently a sign of a dirty HVAC filter, but it is often a sign of it, but my kids are very sensitive to allergens & dust, so when the HVAC filter starts to gather more dust than usual, their dust sensitivities kick it & they sneeze & cough a lot more than usual.