Over the years, I have read *a lot* on this subject - opinions seem to run the range of something like: around 100,000mi. to "I'm over 300,000mi. and I'm not planning on doing a thing until the slack is greater than 5-6 degrees."
For my two 300CD-Ts, I've done it at 125,000mi. Peace of mind in my opinion. You'll probably get plenty of others.