I was amazed how dirty my diff oil had become at 9000 miles from new ( 3.5 years) when I changed it. In hindsight I'd have done it after the first year - I expect it'll stay cleaner after the initial change so its going to be a 2 year thing, like the brake fluid, for me.
The mazda gearbox by contrast had very clean oil at the same time/miles, so I think I'll be doing that every 5 years.
