I was just about to ask about one of the junction boxes I have in my garage.
See the electrician though it was a good idea to run the Romex on top of the
drywall and then splice it with another wire within a surface mounted
junction box. I was honestly thinking the work was crap and then considered
the task of burying the box in the wall so that it wasn't exposed.

Then I read that in fact the junction boxes have to be exposed and in fact
the electrician may have done perfectly to code work. Well aside from the
extremely long Romex he left flopping around in the wall and the handle of
Romex that he kinked into the box, and the fact he didn't ground a metal box
containing wire splices. I'd very much like to get those wires back into
the wall, but since the box needs to be exposed for inspection and work, is
it acceptable to only have the lid of the box exposed? I know they sell
blank cover plates so something like that must be done.

Also when I trim the wires to put them back into the wall, how much slack is
acceptable assuming that in the future I may have to do other work? I'm
thinking 6 inches of romex plus slack with unsheathed wires in the box
itself.

Basically I'm cleaning up the "work" done when they installed my gas furnace
in the early 80's. Basically they removed the oil furnace in the garage
leaving the wire, installed the gas furnace, ran the connection wire 25 feet
to the garage and punched through 2 inches of concrete and 6 inches of sill
plate, spliced it to the old oil furnace connection, which then runs 25 feet
to the breaker box. All I can think of is that the HVAC installer must have
owned stock in a copper mine and the previous owner was deathly afraid of
drywall work.