I can't speak to the black finish, but the silver finish on my 642 has not been holding up too well.
I've had it for about 7 months now. I've fired about 700 rounds through it as well as done a lot of dry fire practice drills. It's been my primary carry gun in an IWB holster for the past few months during some hot and humid weather.
I'd say at most 1/4 of the finish is gone from the backstrap, mostly at the bottom but also at the recoil shoulder. There's some finish gone from the left side of the frame between the grip and the cylinder release. There's also some finish gone from the front edge of the frame and trigger guard, presumably from holster wear while practicing draws.
Most of the exposed areas are just dark grey while some areas look polished.
The only maintenance I've done has been a wipe down with a patch or rag with a couple drops of CLP followed by a wipe down with a dry cloth about every week or so. I also wipe the gun down with a dry rag each day after carrying it.
Fortunately I purchased this gun as a tool and not as a collectable/display item. I knew the finish would wear from my carry/practice routines, but it would've been nice if the finish could've lasted a bit longer. I'll probably get it refinished at some point.
I've considered getting the 442 for my next J-frame, but I think the corrosion resistance of the stainless steel 642 is more important than the durability of the alloy frame's finish.
I should also add that there have been many 642 owners who haven't had problems with their guns' finishes.