This is a technique that I actually do use with my 4-year-old. He can "buy" extra time to stay up reading (2 baby carrots), 3 extra games of splatoon (7 baby carrots), and so on.

I wish I could remember although considering I recall one of the components involved me visualizing time as decrementing hourglass-y down the hallway which lined up with where I was sitting I'm thinking it wasn't particularly rigorous.