I would imagine checking valve clearance would consists of taking off cylinder head cover and timing chain cover to set TDC. If you can do it on the car then the whole think shouldn't take more than 1-2 hours for experienced mechanic. Now, if you need to take the whole engine out to get to timing chain cover, I can see how that could take much longer, but not 23 hours. I need to look into service manual to see what it says about this whole thing, but either way valves should be close to spec for at least 50k miles. If the whole valve checking consists of listening to valve's noise, it's pretty much BS just as much as many other visual inspections. Toyota has non hydraulic valves , Honda uses belts instead of timing chains in interference engines, which means you need to change the timing belt every 100k or risk major valve train rebuild, possibly whole engine trashed, if the belt was going to break.