However, seeing that your cycle is 6 months I might assume that you have a four inch thick filter and you don't want to change it three times a year cause that would cost extra.
I hope at least you start each season with a clean filter. Some don't, they run thru oct/nov with a filter that was cleaning air during the hottest part of the summer and is therefore half plugged up. But hey, furnaces can run and provide heat although they are starved for air. The Duke made movies with only one lung too. Didn't leap over horses anymore tho.
What maintenance is needed? Read your manual and find out what should be done and then call the installer and see what they do. Or a reputable company if cousin George put it in.
Do you ever check the intake/exhaust pipes for several things? Do you check/clean the condensate removal system? The gas pressures? Do you do a combustion analysis? Ever peek in the blower compartment or take a temperature rise and match it to manufacturers specs? Check dip switch settings? Blower speeds? Motors for excessive dust/lint?
The list goes on...