I'm assuming that there you are looking for the number of days between orders placed by the same customer, otherwise the answer is likely 0 since they probably have lots of orders daily.
To do this, you need to set up a report that groups by customer and sorts by date. Next check out Crystal Reports help for info on the "DateDiff" function and "Running Totals." In particular see the section on "Creating running totals using a formula." I think you'll need to set up multiple running total variables to do this. Sorry I don't have more time to explain this but I think this should help get you started.