When is usage still subsequent in a very long document?

We are supposed to spell out a military rank when used before a soldier’s name, e.g.,

Master Sergeant John Doe.

In subsequent usage, we are supposed to abbreviate the title to

Master Sgt. John Doe

Style guides differ on the exact abbreviation with some preferring MSgt)

My question concerns the definition of subsequent. If you are writing a novel or any lengthy document, should you avoid abbreviating titles entirely or only abbreviate when the title appears several times in the same few pages?