Try something like this (this will handle if its submitted ON the weekend. (starting point, I'll have to come back to deduct the weekend time if its submitted on a Monday kind of thing)
var date SubDateOnly=ToDate([Date/Time Submitted]); //convert date/time to date
var datetime NewSubmitted=ToTimeStamp(ToWeekdayP($SubDateOnly), ToTimeOfDay("11:59 pm")); //create a friday night at midnight timestamp
var datetime AdjustedSubmitted=If(IsWeekday($SubDateOnly), [Date/Time Submitted]), $NewSubmitted); //If its a weekday, use the regular timestamp, otherwise use the friday night time.
$AdjustSubmitted-[Date Created] //put it all together
If your submittals are never greater than a week, then you could add some conditional logic to evaluate the start and end, and if the total is greater than x number (i.e. it includes a weekend time) subtract that 48 hours.
