Depends. Assuming you are starting with a fresh change and a trans in good condition with a radiator in good order:
- All highway at gentle cruise - 20,000 miles.
- Mixed city/highway with no lengthy dormant periods - 15,000 miles.
- Mostly city with much stop-n-go and high heat/idle/AC - 10,000 miles.
- Towing/racing/etc. - 5,000 miles.
- Long dormant periods - annually.
- TB and VS mentallity - after each transmission failure.
These numbers are based on my ten years of driving an '86 245 B230F with AT going 265,000+ miles.