We seem to be talking past one another. If your calendar in Project Information is the 24 Hours calendar that comes with Project and your definition of a "day" is 12 hours per day, you should be able to enter 5 and have it span 60 hours.

Ah....wait. Right click on the Gantt chart to show the shortcut menu, and select Layout. Deselect the option Round Bars to Whole days. Does that fix it?