There are actually too many factors to give a specific answer and the simplest way to answer this is by saying… it depends on your individual fish. So let’s break it down a bit.
Put first, an alternative to starving your fish…
Vacation Feeders (Slow Released Food)
If you’re asking this question because you’re going on vacation, you can actually buy slow release foods. (Some include: Zoo Med Betta fish Banquet Blocks and Penn Plax PBV1 7-Day Vacation Fish Feeder) These are really great to have around the house as some of them last 2-weeks and will easily keep your fish friend fed! It’s much better to be safe than sorry. No one wants to come home to a sick, or worse, dead fish! Now, slow release foods aren’t perfect, and there have been complaints that sometimes they release too much food (that will either overfeed your fish and create too much ammonia inside of the tank) and there have always been complaints that they may not release any food at all. Of course, this is caused by factory defects, which is unavoidable, unfortunately. I would recommend trying these feeders out a few days before you leave to make sure it’s releasing the proper amount of food for your fish. It’s also important to try slow feeders in advance, since your Betta may be picky about the types of food it eats… leading us to the next option….
Although a bit more expensive, these feeders can be filled with any pellets or freeze-dried food your Betta is used to. Just fill the compartments and it will slowly rotate feeding your Betta a couple times a day. Some of the top rated ones include Eheim Battery Operated Auto Fish Feeder and Fish Mate F14 Aquarium Fish Feeder.
Factors that change how much your fish needs to be fed
Since our swimmers are cold-blooded, their metabolism correlates to the water temperature. Fish that are kept in warmer temperatures have a higher metabolism, so they must be fed more frequently, as fish kept in colder temperatures have to be fed less. So, since Bettas are tropical fish, it’s safe to assume that they can’t last very long without food (although skipping a few days is fine, especially if you are starving once a week for bladder health.)
If you have a larger tank, your Betta is getting much more exercise. If your tank is around 10 gallons, you may be able to get away with a couple days of not feeding your Betta, but you definitely want to make sure your Betta is getting the nutrition it needs to survive. Remember “surviving” and thriving are two very different things, and your Betta will live a longer and healthier life if given proper care.
How long does it take for a Betta to starve?
Bettas usually begin starving around 5 days. This is when the first signs of survival mode kick in and the Betta will begin using its reserve to stay alive. Wild Bettas are usually surrounded by food, and so they usually don’t have to starve or wait to eat. With this in mind, it would actually benefit your Betta to eat a couple times a day, similar to feeding other animals.