2hours to swap out a turbo exhaust, that requires intercollers moving too. so id guess at no more than 2 hours??? for yours given its a larger system in an NA car?
You'll probably find the exhaust flange bolts are severly corroded and will need cutting off with an acetylene torch so not as easy a job as you think Rear PU will probably need to come off?
2 hours if everything is free, up to 4 hours if the nuts are all corroded, and you may well end up needing a new lambda sensor as one always refuses to budge
Don't know where you are but these people in Hayes sorted out my Gundho hack and Sid torched off those pesky bolts - £250 all in. Am sure they would fit any exhaust in the same way.
Design 911 quote 3 hours to replace the mufflers including using a new clamp/bolt kit. My local Porsche indie has quoted 2 hours for the same. Both quoting £70/hr + vat
I suspect the indie would go into a 3rd hour if it was a bh but he does charge by the half hour to.
Zorstec in Shipley have done quite a few 996's mine included. As a 911uk forum member there were quite a few who drove over to have the Oettle bypass done for circa £180 all in.