I feed all my Bettas twice a day, 3 pellets in the morning, 2 at night.
Boris is just like the rest, always hungry, eats everything, never leaves leftovers. Yep, that's every Betta, young or old, pigs when it comes to food, shameless beggars. I worry if one of my Bettas isn't begging for food.
As for duckweed, I doubt your Betta could eat enough to make a difference. Bettas can eat plants, but it's not like they're going to clear out a whole patch of duckweed in a day.