Depending on how often you use your heat unit, you should clean it between 8 and 12 weeks. It is best to have it serviced by an HVAC professional if you do not feel comfortable doing so yourself. There are also a few things you can do as preventative maintenance to help keep your heat unit running fine.
During the winter months, check for ice and snow build up on the outside parts of the unit. Make sure they are not blocking the airflow into the unit.
Keep foliage about 18 inches away from the unit to avoid debris building up.
Make sure your unit is 6-8 inches off the ground and away from gutters with standing water.
Clean or change filters about once a month.
Make sure registers are open and clean coils whenever they appear dirty.
During the hot season, keep your thermostat above 65°F and in winter months above 70°F to keep your energy bills low.