I stand to be corrected here but in my experience it would take one person more than 3.5 days to carry out the LAMS Annual inspections on a PA28.
Just to do the engine inspection would be the best part of a day alone (ground run to ground run, cowlings on and off, oil, filters, compression check, exhaust, ignition timing etc etc).
As for the airframe that includes removing all the inspection panels, interior trim etc and refitting it after. Jacking the aircraft up (and lowering it after), removing all the wheels, regreasing wheel bearings, battery, instruments, filters etc etc. And you can't inspect anything until it's clean.
Then there is the paperwork.....
What you can't do is divide the total cost for the inspection by the hourly rate and say it is going to take x hours, iot never works out like that. Most maintenance companies give a fixed price for say fixed gear or retractable aircraft. Some aircraft are more labour intensive than others (Chipmunks, Bulldogs for instance).
If anyone claims they can do ALL the LAMS Annual tasks in 28 hrs their either very efficient or not doing it as well as it could be done!
Aircraft are labour intensive beasts, much more so than cars. The Annual inspection for a light aircraft isn't that much more than a modern equally expensive car, or am I not really in the real world?
Standing by to be corrected!