365 days a yr / 4 (a 24/72 has you working one day out of every four), x 24 (hours), / 52 (weeks in a year)
365/4 = 91.25. 91.25 x 24 = 2190 (your hours worked on the year). 2190/52= 42.11.
They're working 42 hours a week, on average. If they're FLSA, hours come off some checks, and get added to others. Otherwise, you'll see a small chack, a medium check, and a large one. This is more obvious with a 24/48, or a modified Detroit(?), WOWOWOOOO (W=work, O=off), which is the same as a 24/48 but structured differently.
A 24/48: 365/3 = 121.6667. 121.6667 x 24= 2920. 2920/52 = 56.15.
If you do a 24/72, you're working an average of 42 hours a week. If you do a 24/48, you're working a scheduled 56 hours a week.
Edit: There are several threads devoted to the discussion of whether it's acceptable to be POC on the overnights, or whether you're entitled to be paid for every hour worked, whether or not you sleep or otherwise have significant downtime.