Most people feed theirs once or twice a day. I give mine at home 2 or 3 pellets each in the morning, one or two in the evening, and fast them on Fridays. My office fish gets fed one pellet a few times a day, and fasted on the weekend. Is the bettamin the flake form? If so, you might consider branching out to other varieties such as Hikari Bio Gold pellets, live tubifex worms, baby brine shrimp. There's a lot out there.
They should have one staple that gives them all-around nutrition, and that would be the flakes or the pellets. Worms and shrimp are treats and yummy, but not so nutritious.
For now, feeding flakes while keeping in mind that their stomachs are about the size of one of their eyes should be fine. With pellets it's easier to determine the amount of food you're giving them.
Bettas live about 2 or 3 years, but in some cases longer. Petstore bettas are typically around a year old, but since yours is small, it could be younger.