Time estimates are funny things.
If the car is new enough to easily support a remote starter, (mine is not) the 1/2 hour install estimate is probably correct 90+% of the time. (Especially if they're using fuse taps, crimping instead of soldering, etc.)
The onsite folks are likely charging more for onsite service, and (like most people who do deliveries or on-site work) scheduling with a best case scenario in mind, so they're guaranteed a full day of work even if somebody is a no-show or they have cancellations. This is why the cable guy is usually late, but sometimes calls you and say, "hey, I can come 4 hours early - you around?"
The shops are telling you 3-4 hours because that's a worse-case - it's to manage your expectations. They'll schedule just as many installs per tech as the mobile guys, but they'll never have to tell a customer, "well, we're running behind, so umm... about that time we told you before..."
The techs can do their thing, take an average 20-40 minutes per install, take their cell phone to the bathroom for a nice long squat, spend 2 hours on that one stubborn car with "oh crap, that thing", and the counter/reception staff still have enough time to flirt with UPS driver and hit Facebook before they call the customers and tell them their car is ready.
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