My estimate would be 4-6 hrs to repair.
Porsche uses strings to keep the top taught and sliding inside the bow and uses a wimpy cloth band to hold the strings in place. The band loses its shape and lets the string loosen the taughtness of the top, which allows it to slip over the outside of the bow. Pretty rinky-dinky system. If you raise the top around 10", you'll find a velcro strap inside the interior in the area of the bow. You can undo the strap and see the string / band setup. According to the Service Mgr I talked to, you can loosen the string with a screw further back on the top and tighten the string. That area looked a bit to cramped for me to deal with, so I tried tightening the band. My attempt lasted about a week and then it went back to slipping over the bow.
I didn't ask him how much it would cost since I thought I could conquer it myself.
Be sure to look at the seams of the top. Leaving it with the top over the bow can really stretch the top and stress the seams.