Every 1-2 years or 15-20k for dino fluid.
Ever 2-3 years or 30-40k for a quality synthetic.
ATF fluid (original spec) needs to be changed significantly sooner than a synthetic gear oil (new spec) does.
I have changed mine 3 times in 3yrs/31k. 500-1000 miles (cocktail), 16k (cocktail), 31k (Redline)
I would recommend changing it more often if you drive it hard at all. Both times I changed the cocktail at 15k it was pretty harsh looking. Not surprising for an ATF used in a manual tranny.
I also highly recommend the Redline MTL.
