How often should I clean my Betta's 3.5 gal (U.S.) tank?

My son was gifted with a beautiful scarlet betta for his 2nd birthday, and I know that these fish can live from 3-6 years if cared for properly. So I want to make sure that I'm doing it right. I found alot of info on websites, but not any about how often my tank should be cleaned. After we recieved the fish (in a totally unacceptable gl cube the size of my fist) I read up on them and bought him a new 3.5 gal tank w/ a heater, thermometer and filter. I used smooth natural pebbles for substrate and put in a few long, leafy silk plants. I bought the betta pellets and the bloodworms and I've been making sure that his tank is a comfortable 80 degrees Farenheit. So, do any of you know how often I should clean the tank? Any other suggestions are also welcome.