They need to be serviced once/year. You can do the heat in the fall, and the a/c in the spring, or do them both at the same time in the spring. It has to be warm enough to run the a/c.
As far as filters go, you will find your own timing. I have three returns in my house, and all three filters are on a different schedule. Once a month is the easiest, and works best for the most part. However, you may find that your filter gets dirty in 3 weeks, or it may still be clean at 6 weeks.