Bettas will over-eat in an instant, as you've found out. The betta's stomach is roughly the size of it's eye. So a lot of people gauge how much they feed by the size of the eye. You can feed twice a day. Many people fast their betta once a week- so no food for one day.
Like Aiken said, he may bloat- just keep an eye on him. You can always try to give him a pea if he does bloat.
Platys, Tetras, Mollies, Bettas, BN Plecos
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