i have a 94 sc400 with 150k on it and the timing belt was done at 90k in 2002, the dealership said they recommend it be done every 90k or 6 years. Should i just wait another year or so to do it since its only got 60k on the old belt or should i do it soon since its been over 6 years?

Do it when you can. While it's all out that is the perfect time to do everything else. Front crank seal, cam seals, water pump, timing belt tensioner, and a few others but those are the main ones.

I believe most of the dealers will say 85-90k is the interval, but if it's sat awhile that belt can be ruined if it sits in one spot too long.

you can also take your cover for the timing belt and inspect the belt to make sure there isn't any cracks on it.