For what it's worth as a single data point, I preemptively replaced my fork seals, bushings, etc at 67,000 miles. It really wasn't necessary; nothing had been leaking, and the bushings still looked good. If I hadn't already bought the new parts, I would have been fine just buttoning everything up with the old parts in place. My riding is a pretty good mix of pavement, dirt, and gravel.
I do replace the fork oil every two years. Maybe that contributes to the longevity of the bushings, since any particulate matter in the fork oil gets removed regularly during the oil changes.
I suspect that the fork oil darkens, at least partially, because of heat. I have no idea what the temperature is inside a fork, but oil being constantly compressed and forced through tiny passages has got to generate some heat.
Here's my McGuyver fork spring compressor, made from a cheapie Harbor Freight bar clamp, a trigger clamp, and some aluminum bar stock...