Prime is good if you need a little help between water changes, as it will (somewhat) detoxify ammonia (into ammonium, which is less, but still toxic).
Some people say you need a 100% change weekly, or bi-weekly with a 50% every week...I don't buy it. Large water changes stress fish - pick up a test kit and you'll get a real idea of just how quickly your water gets to a point that it needs to be changed. I will say that 50% every month in a tank this small is a big no-no, though. It would need to be considerably more frequent than that.
I keep a betta in a 5 gal filtered hex, and I typically will perform a 25% change weekly. Your mileage may vary, of course, based on how well cycled your tank is.
Any questions, just let us know.