Most GM autos recommend changing the transmission fluid every 100,000 miles if driven normally, or 50000 if driven harder. driven harder relates to distances less than 20 miles one way, lots of town driving, towing, hills, etc. Look in the owners manual. What I recommend is simply changing the transmission filter every 50K. By changing just the transmission filter you are replacing 2/3 of the fluid itself and not having to pay $100.00 for a flush. There are numerous threads about dislikes of having tranmission flushed done by "quick lubes". By not doing either or, you do put your transmission in a lot of danger, the transmission is just like the engine, it likes fresh fluid and a clean filter. It pays in the long run also, i have several GM's with over 200K and I have never had to replace a transmission.