Q: How many man hours is required to change the spark plugs ?

Just to replace the spark plugs is going to require about 2.5 hours to remove the upper intake manifold and all the coils to access the spark plugs. If doing the job for a misfire then be prepared to replace oil soaked coils and coil boots along with the valve covers that are leaking oil into the spark plug holes. This would all be with additional charges for parts and labors if found to be the cause of the problem with oil getting in to the spark plug tubes and on the plugs and coil boots. The valve covers would be an additional 1.5 hours and the coils and coil boots about an additional half hour per side of the engine. Any labors quoted does not include diagnostic charges to find a problem. Spark plug replacement is considered a maintenance item and in most cases are not the cause of a misfire but the end result from a problem that caused the plug or plugs to misfire.