Depends, if a driver idles overnight, that's 8-10 hours. If he idles waiting to get loaded/unloaded, that could be 1 hour to 8 hours. Parking for a few minutes to sign in at the shipper and idling is a few minutes.
In hot weather, you might idle over night, but turn the truck off at 3 am when it has cooled down. And another thing, idling is normally done at about 1000 rpm to keep the engine lubricated.
Some drivers have their engine running basically 23 hours a day, including driving and idling. Jimmy