Umm...I missed out! When I installed lift kits, 50 hours would have handled at least 2-1/2 vehicles...The 18 hours would be reasonable for a thorough job, from installation of parts to full alignment of axles, wheel alignment and such. I think you're right, though, Biggman100, this could be a loaded question. In the forums photo gallery, owners can comment on their individual experiences, and this might be insightful.
I do know that installation shops, familiar with particular lift kits on specific chassis applications, do quote labor from a "menu". For example, there is a 4-Wheel Parts retail outlet with installation shop at Sparks, Nevada (and elsewhere), and they must have a formula. We might look into that angle.
Similarly, auto dealerships, like a Jeep or Ram truck dealer selling Mopar lift kits, must have a formula and work from an estimate menu. I'm not clear how New York State handles this, but most states have a mandatory "quote" requirement before a job begins. This would be impossible to do unless the shop has a clear sense for the labor time. An independent shop might have more latitude for "time and materials", but I doubt it.
In states like California, an agency like the Bureau of Automotive Affairs insists on an initial estimate, though there is a provision for bumping the charges (with the consumer's permission) if a problem or additional work emerges during a repair. This generally applies to repair work on mechanical or electrical systems where the job is unclear before assembly teardown. Visualize an automatic transmission, engine or a transfer case, where the parts cannot be inspected without the teardown.
There is also the lift kit manufacturer's "estimate" of installation time, which also applies to many other aftermarket automotive products. This is typically intended to encourage consumers to purchase a kit, and it's not clear what the labor time reflects. (Is this the time it takes an expert or team, with all power tools available, to do the job at a professional shop facility set up for replacing these parts? This is hardly a home garage or under the apartment carport!) As Megatron aptly noted in his reply to our project difficulty rating topic, even the tools you own can determine how long it takes to do a job. Your familiarity with a given procedure is a whole other story!
I did open a can of worms...so let's keep this question open!