First of all, you will need some parentheses to make the expression you have calculate correctly. It should be:
"Incident Duration: ([Incident End Time]-[Incident Start Time])*24*60"
Of course, I'm assuming the start and end times in this formula are cell references.
Secondly, you must format the cell to display the result not as a time or date, but as a simple number (e.g., "General").