OK, if you are assuming a full 72 hours, then you have a base of 21,600 (100 x 3 x 72) man hours per half hour you play.
Yeah, I suppose you could interpret it that way. I guess I am just too old school where the 1st Ed AD&D DMG states 100 men labouring for 3 days. Since men don't labour for 72 hours, I always interpreted it as a work day. *shrug* Whatever floats your boat.
While I love the concept of this item, and indeed my Bard is making one, it is an item that needs to be carefully discussed between DM & player before it is used in a campaign. It is _so_ cheap and the language used to describe it's affects is so vague that it is very easily abusable. Why wouldn't every kingdom with a Bard and a wizard have a couple fo these sitting around?