The amount of time also depends on the particular cartridge, as some cartridges take longer than the 20-50 hours you state. Koetsu is certainly one of those that take at least a hundred hours or so to break in. Both my Koestu's, the Black and the Rosewood Signature, while doing the majority of breaking in during the first 50 hours, continued to change their sonic characteristics up to at least a hundred hours (and possibly even after that, but I was too happily listening to notice at that point!).
The affects of the breaking in on the Koetsus was pretty obvious, as both the treble response and to a lesser extent, the bass response, became more fully extended. Also, a less obvious improvement, (a refinement actually), was to the soundstaging and imaging.
And to answer you questions, the break in is very gradual, (not abrupt), stylus wear is not a part of it (the suspension is actually what is being broken in), and no, the resale market value (like virtually everything in this world), goes down the more you use it.
My two cents worth.