Most installers who have been in the business for longer than a few years charge by the sqft, not by the hour. Of course they include difficulty in the sqft... We charge more for a bumper than we charge for half a vehicle.
Theres a guy I know who's been in the business for 20+ years, started doing wraps when 3m first introduced them. He actually instructs the 3M courses now. He can do stuff in a quarter of the time most others can.
He Competes in speed wraps...and can wrap most a vehicle in an hour and it looks better than most the hack jobs I see on the streets obviously real wraps he doesn't rush like that, but still.
He once told me if he charged by the hour instead of job.. He'd be undercutting competition by half.
It's the same for people who buy a flatbed. Just because I can print
signs faster, and cheaper than someone with a roll to roll... I'm not going to drop my prices by half.
One in getting at is one method doesn't suit all. You need to see what works for you and your pace. If hourly is higher.... Charge hourly. If sqft is higher... Charge per sqft.