It's somewhat objective because its really how you drive it. I know people that have over 80k on a stgII 4eat no problems, others break their tranny earlier, usually because they brake launch, drag and otherwise are hard on it and don't maintain it.
I have a 4eat fxt and just swapped in a VF43 after being at Stg II for the last year (12k). I added a tranny cooler which I think is a real good investment. They are relatively cheap and keeping the tranny cool is key
Whenever you upgrade there is always a chance things might go south but overall you should be fine provided you maintain yourcar and your driving habits are reasonable