How long does it take a repair shop to weld the catalytic converters?

it depends on the exhaust setup and vehicle type. cats on a suburban should be a few hours. typically you cut out the old ones, tack weld in the new ones to set location. then you take out the merge pipe the cats are on and fully weld the cats, install new gaskets and reinstall merge pipe. and if the vehicle is driven in, add some time for it to cool so 1200* cats can be removed.