One Piranha would need probably at least an hour to fully eat a human by itself.
However a school of 500 Piranha fish could easily eat the flesh of a human in just 5 minutes because there would be more Piranha fish to devour the flesh of the human more quickly.
One Piranha fish really wouldn't have room in it's body to hold all the flesh it would eat off the human so it would need more fish to eat the human.
Piranha fish are dangerous though and my dads friend had the flesh eaten off his hand before in some water years ago.