Typical flow rates for HVAC systems are ~1 air change per hour, but to effectively filter the air faster than the dirty air leaks in, you need about 5 air changes per hour (AHAM recommendation).

If using the HVAC, run the fan continuously, and ideally use the 1" 3M 1900 filter. Other brands or models (including the supposedly 'better' 4" 3M 1900 filters) have fewer pleats, which works out to smaller surface area and worse airflow resistance. Check BJs website, usually they're $10/filter in a 4-pack.

Typically these need replacement every 3 months. The only way to really mess it up (and possibly break your HVAC system) is to completely forget to replace the filters, so I try to set redundant phone alerts with different family members.