I am new to Betta ownership and had the same question you do. Everything I read said only to feed as much as the fish will eat in 5 minutes. So I think this amount depends on the size of the fish, how good of an eater it is, etc.
First of all, I was feeding mine way too much and there was old food at the bottom of the bowl.
What I did was I skipped the day of feeding, then when I did feed them (I have two, in separate bowls) I put two little pellets in at a time. And watched how quickly they ate. Kept giving two at a time until they stopped eating. Which for one was indeed within about 5 minutes, but the other one is just a slower eater. But after 15 minutes I remove the remaining pellets from the water - and they are usually still floating at the top.
I think if you would try this, you will determine the eating pattern of the individual fish.