Like Gary says, there is no magic way to have it. "stop counting".
Rather, you are calculating the duration between two dates and therefore you need to know when the start date is, (that is easy because you have that field) but the question is when the end date is. The end date is either today which keeps moving or another date that needs to be recorded somehow as an end date.
So either you need to change your field property to be an Append Only" and log changes, and therefore you will get the end date from there, or else you will have to have a Pipeline run to detect when the record is saved, and the status changes to a certain status and records the current date into a field.
My suggestion is to make your status field be set to Log changes.
------------------------------
Mark Shnier (Your Quickbase Coach)
mark.shnier@gmail.com
------------------------------