You could probably do this with a form rule and a formula duration field. Basically the form rule would update a date field when the status changes called Date of Status or something. Then in your formula field you can either compare that date to today or the date of status to today.
If status is complete then it should get number of days between the date is created to the date it was marked as completed otherwise it should show the number of days since the date it was created to today.
If thats correct logic try the code below.
Code not tested
IF([Status]="Complete", ToDate([Date Created]) - ToDate([Date Status]) , Today() - ToDate([Date Created])
