1,800 miles don't even think about it anymore. I have the extended warranty so I factored a tranny flush into the price of the warranty.
So what I mean is...
I will drive the car to ~100k miles. If I didn't get the warranty I would have the fluid swapped at 50k miles. The cost (when I called the dealer) was about $250. Since I got the warranty up to 100k miles I won't have it flushed and 'saved' $250 off the price of the warranty (mentally of course
). The manual states that the fluid is lifetime so to remain under warranty I don't have to do anything.
To each his/her own. That's just what I'm doing, not a recommendation.