If it's and auto I would say every 10-15k miles, especially if the last time it was changed it was only a single drain and fill (2.5qts). As a single drain and fill only changes around 1/3 of the tranny fluid in the transmission. So I would say have it done soon if possible . If you do a search you can read my post on the SS Auto tranny's(If you have a 5th Gen Lude).