I think the 48mm WP USD's have 3 bushings, rather then the two the 43mm forks have, but this might only apply to the 525. (I just mention this, as I would think a "3 bush" fork would suffer less wear, and it seems the bushings and seals are the only things I find worn on the street forks I've rebuilt.) I've no practical experience to back up the following, but my plan with light offroad, non-competition use, of say 5-8k miles a year, was to flush the fluid the first year, and do an inspection and rebuild the second year. I would sure want the WP fork and shock manuals if I was doing the job, I snagged the CD off eBay. (Hmmm, that would be a good place to see about the bushings.)
What about the shock? This is were I've heard more feedback about a noticeable improvement after a "rebuild." I just wish there was an easy way to confirm the shock is holding it's nitrogen charge.
PS. I'm thinking about increasing the fork spring and shock spring rates on my 640 ADV R, and will be posting another thread to that end in the next day or so. The more feedback the better. Cheers, Mack