My toilet refils every 20 minutes. My floper is new.

Ur flapper is not sealing correctly or may be the wrong one for your unit. Check seal and reset flapper.

Chuck

I agree, the flapper may be new but it is not sealing. Most flappers are universal but it still could be wrong.You could also have a rough or nicked edge on the flush valve seat. However, lets start by you telling us why you replaced the flapper to begin with.

If you still doubt, then add some food coloring to the water in the tank. You will no doubt find, within a few minutes, that the water in the bowl will turn the same color.