Toilet does full-blast, one-second refill every hour or so
Add a few drops of food coloring into the tank after it has refilled before it kicks in again.  Mix it up a bit to make sure it is dispersed.  Then watch the bowl to see if any of that colored water leaks though.  If so, then you know for sure that you have a leaky flapper as others have described.  If not, then the issue is probably on the supply side.

Note, you might want to lower the level temporarily, just to make sure any of the colored water doesn't go down the overflow in the case the supply is putting the water in and it is not leaking.