I haven't done enough to have an average, so I can share is my limited experience.
With that said, someone who knows what they're doing and with all of the tools and parts should be able to set up a 9" from scratch in less than 2 hours IMO, but I've seen guys do it in 20 minutes or less. If you need to clearance the case, drill holes for an ARB locker, rebuild the Trac-Lok, etc, then add some time. Before I did my gears myself, the shops I called locally were quoting 1/2 to a full days labor for a 9", which seemed high to me at the time and still does, but I get it...they're a business and need to cover their overhead. Plus there's the liability of something happening and having an unsatisfied customer due to gear noise or something that may be more related to the gears themselves than to the quality of installation. Paying someone else $500 to $900 to set up gears seems silly when you can buy the tools and do it yourself for less.
FWIW, most of the shops I spoke to would have preferred to supply the gears and master rebuild kit themselves, that way they know what they have and are typically confident in the quality. Some modern aftermarket gears can be very difficult to get a good, usable pattern.