How about change the fluid, and see how it looks. If it's nasty then rebuild. Though it could very well look nasty and be fine, but as soon as you see ugly fluid coming out, it will be suspect in your mind, right? At that point I doubt I would be happy until I saw what the internals looked like.
I still say change the fluid every 6 to 12 monthes, and rebuild every other year. If it's just sitting, very low mileage I'd go longer, if it's ridden hard and/or lot's of miles, I'd rebuild annually.
There now, it's all clear as muddy fork oil, isn't it?
If your riding the ADV as hard as you should have been riding that 400 SX, your more of a man then me!