As you've read, the factory spec is change at 30K and the gap is .045. The plugs in my Hemi are less than 1K from 40K and still doing well. At a whopping $2 a plug, when the time comes I plan to change them all. Some folks feel since the secondary plug only fires on the opposite side of the engine during that cylinders exhaust stroke, they can last longer. I say when you go in, do em all!
There are 8 plugs in each head with coil packs mounted over 4 of the 8 on each side secured by 10mm bolts if memory serves correctly. Dealers are rumored to charge $250-$300 for spark plug replacement on the Hemi as it does take 2-3 hours. You'll want to have a good selection of extensions and it'll help to remove the air hat and associated tubing to the air box to give you room to work.