Depends what they are doing and when.
If they are just replacing the seats, it might take 3 weeks.
If they replace the seats at the same time as doing a D check, when they remove the seats anyway, then it wouldn't take any longer then the D check would normally have taken anyway.
When AA added "main cabin extra", they were adding it to their narrow bodies overnight (pull out 1 row of seats, realign the rest)