Depends on the temp but 2-3 days. When my Bettas bred I thought I lost the batch. After the mating occurred I saw nothing for weeks and figured I lost it. But then one day I saw this tiny thing in my tank - and I mean TINY. It was just a speck like one of those things that live in the algae along the tank walls. After close inspection I realized I had a Betta because it was way too small to be a new born Guppy.
What I'm trying to say is that you may not know when your eggs hatch especially if there are lots of plants and hiding places in the tank. I ended up with 50 babies from 4 spawns. Surely some of them were eaten but as long as your adult fish are well fed and you provide hiding places you'll end up with some. Also I wouldn't worry about feeding the baby Bettas. During the first weeks of life they eat stuff in the tank that you can't even see. If they're separated out into a grow tank Microworms/Walterworms/Vinegar Eels are a great first food. If you're here in the US the guy from Aquarimax.com will send you a starting culture for cheap. He's also very honest.
I'm not sure if the male Betta eats fry or not - the female is a different story. I had several other fish in the tank and I suspect it was them. Its also possible that none were eaten and just 50 Bettas survived to adulthood. If it was me I would leave him because he catches and spits the newly hatched fry back into the nest when they fall out. I'm guessing they need air (the bubblenest) the first few days of life.
How long do they depend on the male? Not sure. I'd like to say a week after they hatch but I'm not sure and don't want be responsible for killing your batch. You'll probably have to rely on google for that info. Sorry I can't be of more help, but I couldn't even see the bubblenest in my tank because it was so well hidden. But I have to say my male was a great parent and I doubt very much he ate a single baby - besides they're way too small for him or any larger fish to be interested. They're a fraction of the size of newborn livebearers.
You'll start to see color in about a 4-6 weeks. After a month the babies spend a lot of time in the bottom of the tank hunting for tiny creatures in the dirt. At this point they might be vulnerable to bottom/lower level fish in your tank.
Generally, you want to remove the male when the fry are free swimming. Some people leave the male in the tank for the entire rearing process, some not. It's most common to remove the male once fry are free swimming. He is needed prior to this as he needs to tend to the eggs and wrigglers, and keep them in the bubble nest.