All engines differ on idle vacuum - injected, carbureted, cam selection, ignition advance, rpm, number and size of emissions and working vacuum bleeds - all makes a difference. A mildly radical, lopey cam absolutely destroys idle vacuum.
That said, idle vacuum in a normal road car V-8 engine at about a 650 idle speed should run from 18" - 22". There is practically NO airflow into the engine at idle speeds and very little at highway speeds, steady running. In my Northstar at 65 mph the throttle is only open about 9%.
On the road, the higher the vacuum for a given engine speed/load - the better the fuel mileage.