Well its a bit of a debate. Some say if you're only feeding once day then no, others say you need to fast them no matter what. I typically fast every Friday but I have skipped fasting from time to time. I feed once a day so mine always go 24 hrs without food & when I fast them its 36-48 hrs, on the weekends I tend to feed them earlier in the day & sometimes more than once in a day. As with a lot of things in this hobby what works for one may not work for another.
I don't have a set fasting time.. most of mine are fed 2-3 times a day and no skipped days. But some of my adults I will often not feed them for a day or two depending upon what size container/tank they are in. Feeding higher quality foods you can feed less, as more nutrition is taken in, lesser quality food you will want to feed a little bit more to meet their nutritional needs.
In the wild they eat when food is available.. sometimes it's a lot per day, sometimes they skip a few days. But since these are domesticated I say no need to skip a day if you don't want. It won't hurt to skip a day or not.
I don't fast, at least not my female anyway. Sometimes I fast my male because he's prone to constipation and if I go to feed him and he doesn't acknowledge my presence or swims away, he doesn't get fed.
My female on the other hand, if she misses a day of feeding, she shreds her live plants to pieces, and then if she sees me nearby, she waits at the lid opening, and will actually start to splash around and jump. I've had her jump completely out of her tank before on her never-ending quest for food, so I've decided for the sake of the plants, and her safety, I won't skip a day of feeding.
One thing to keep in mind about fasting is that it ensures that occasionally the digestive tract is completely clear. This means that internal parasites are much less likely to find a foothold in the fish. Some angel breeders swear by fasting, because angels are so susceptible to internal parasites, and fasting regularly once a week has been proven to help with that.
I am glad to see this discussed, because I've been wondering about it, too. When I got my first Betta, I was nervous about this, because I intended keeping him at my office, and therefore he would be alone 2 days per week and I would either have to come across town to feed him on the weekends, or take him home. That's when I learned that they can actually fast for a few days, so that is what I have always done. During the week, I feed him twice per day most days, Omega One pellets in the morning, and then a frozen bloodworm or some brine shrimp in the afternoon. On Friday afternoons, I do his water change, and then I give him a pellet or two just to tide him over for the weekends.
My Bettas at home I feed usually twice per day (they get New Life Spectrum pellets, as well as frozen bloodworms, brine and mysis shrimp, and frozen beefheart) and don't fast, but I do feed them more lightly at each feeding, sometimes only one pellet each. The baby (he's not so much a baby anymore, but I think he's still growing) is a little different ... sometimes he gets an additional pellet or two at a third feeding, while one of my female crowntails has a tendency to blow up so where the others get two or three pellets, she will only get one, no matter how much she begs. And I have often skipped a feeding with her, too, if she still looks plump from the most recent feeding.
I also am glad to see this discussion. I have 4 bettas at my office, all in 1 gal glass canisters. I change the water twice a week, feed twice a day (hungry little buggers) and give them extra on Fridays. Monday morning I am the main attraction, as everyone is frantically showing me how starved they are. I was worried that two days fasting would not be healthy for them.