You could go the whole 1000 without cooling it down. Overheating the tranny isn't really an issue when it comes to highway driving. It is more prevelant when it is a hot day, around town driving, beating on the tranny.
Everyone plays the trannys up to be terrible, but they really aren't that bad. Just aren't meant to be beaten on. As long as you change the fluid regularly, you should have no problems. A cooler is always a good idea, I still have yet to do it in the over 50k miles Ihave put on my car. No problems here(knock wood).
The SVX is a true highway cruiser, have fun
