dry food in a jar or frozen food in the freezer doesn't go bad. But uneaten food that sinks to the bottom of the tank will rot, make the water cloudy, and grow bacteria and fungus that make the fish sick. Don't feed the betta more that it can eat in 1-2 minutes.
Like most fish they can survive a famine of a week or so but they will loose condition if they are not fed regularly. The biggest killer of Bettas is a lack of water quality (Insufficient filtration and water changes) not a lack of food.
About 5 months.
how long can a tutle go with out eating
long periods
10days
A wolf spider can go without eating for about 120 to 130 years.
Long Periods
its beauty and long flowing fins
3 years
2-3 inches
6days
Male bettas have a long tail fin, and they are brightly colored; more so than the female which is a dull color. She has a short tail.
6 months
5 days