That depends on the number of cylinders, valves per cylinder, and so on. Your parallel twin is probably easier to service (accessability of valve covers, valve mechanism). Some designs require the removal of the gas tank and even camshafts (eg Suzuki V-Strom).
(Old) Ducatis are also a pain with the desmodromic valves (i.e. springless valves). Some of their valves have 0,0 clearance and the mechanic needs to hone down shims to an exact fit. Also the reshimming needs to be redone on older Ducs after only a few thousand kilometers/miles, or annually.
In contrast, BMWs older air/oilcooled boxer engines are a joy to service, easiest accessability (cylinder heads stick out sideways), only 3 tools needed for adjusting valve clearances (two types of Torx/allen keys and a feeler gauge set), 20mins of work or less depending on your routine.
MY 90 minute number includes pulling the fuel tank, valve cover, camshafts and other body panels from my Suzuki. It's pretty quick if you're familiar with the procedure. I can have that bike torn down with all body panels and subframe off in about 10 minutes if I'm in a hurry.
Ugh, pulling and especially re-inserting the camshaft shouldnt be done in a hurry. At least from my experience with the V-Strom's V-engine. Needs to be cooled down and insertion of the camshaft needs proper care (no dinging of the bearing shells) and one must make sure to not skip a tooth at the geartrain/chain.
I mean, I take my time with the cams, but it's not rocket science. There's timing marks to help line everything up and I snap a pic with my phone before taking them out.
(Old) Ducatis are also a pain with the desmodromic valves (i.e. springless valves). Some of their valves have 0,0 clearance and the mechanic needs to hone down shims to an exact fit. Also the reshimming needs to be redone on older Ducs after only a few thousand kilometers/miles, or annually.
In contrast, BMWs older air/oilcooled boxer engines are a joy to service, easiest accessability (cylinder heads stick out sideways), only 3 tools needed for adjusting valve clearances (two types of Torx/allen keys and a feeler gauge set), 20mins of work or less depending on your routine.