24-11-17, 05:09 PM
How long is a piece of string, depends on your weight riding style etc etc, I would start with looking at the sag rather than how many threads should be showing, I can't remember totally (no doubt someone else will come forward and educate me) but around 20% static sag would be a starting point, go for a ride and see how it feels and go from there.
I have a BMW shock on my Gen1 but with a Ohlins Spring supplied by MCT for my weight and riding style , I'm a fat bastard (18+ stone with all my riding kit on) with a moderately fast riding style. so can't say more than I already have on your situation.