I'm of the "Let The Broody Do It Herself" school of thought. They seem to have a remarkable ability to gauge just how long to time their daily breaks depending on the outside weather conditions. They don't move as much so they don't need to eat as much. While on the nest they get in the Broody Zone and may not respond to treats being offered. I just watch them carefully and if it seemed they never got up or ate/drank -- evidenced by no broody poops, their dishes staying filled -- then I might intervene. They sometimes lose a bit of color but should still be strong enough to give you a firm Stink Eye if you disturb their set.
I don't think the hens keep count of the days, but probably begin their lockdown when they feel the chicks moving in the eggs, and hear them peeping. It's sweet to hear the hens clucking back to their babies.