The fluid starts out life red in color. If it gets burned it turns brown to black. If the color is not red, the fluid loses its viscosity and must be changed (complete flush). On many cars, such as Nissan's, Toyota's etc., the fluid will stay nice and pink for many miles and may never need to be changed. On my 1999 XC, this is not the case. Many will say its due to the tires, but the fact of the matter is the AWD design was rushed to market and the transmission for 2 wheel drives is not strong enough to run AWD and therefore gets hot and burns the fluid. I learned 2 things the hard way (transmission failure at 66,000 miles). 1. When you take your car to the dealer for a XX,000 mile service and the book says "check fluids", the dealer does not check the fluids. 2. My car turns the tranny fluid brown rather quickly (15,000 miles). To answer your question, pull your tranny dipstick out and check the fluid color yourself and flush it or have it flushed at the first signs of discoloration.