Poorly installed ductwork may invite more airborne pollutants into the system as well. Both of these examples can potentially reduce the time between filter changes. Pets: If you are a pet owner, the number, size and type of animals in your home might affect timing between filter changes...
Two local friends exchange witty banter even as they build custom ductwork for a current project house. This was a bigger job as the original 1910 house had no real ducting at all – we had to open up walls and run pipes to every room in the house. Your furnace upgrade will be easier...