As long as you put the date of the customer's last visit in a date field,
Get ( CurrentDate ) - DateOfLastVisit
Will compute the elapsed time in days.
But this needs to be a calculation field set in storage options to be unstored. And do not use a number field with an auto-enetered calculation for this either. If you don't put it in an unstored calculation field, the calc will not update with each new day. (Of course you can also use this expression in a script step such as Set Field...)