I don’t have any data on drill stem life. I do know that several refiners keep a few spare middle sections of drill stem and the top and bottom sections with the flanges or unibolt to minimize the downtime to replace a broken drill stem. I would wonder if your drill stem guide lacks lubrication or has some sort of defect. I have heard about more failures due to bending (especially for welded CS stems), and breaking (typically for 4140 threaded stems) during coke bed collapses.
Is the drill stem still relatively straight? If not it may be getting excess bending force applied to it during cutting cycles. I would also question the type of drill stem guide that you are using. If the reduced diameter is only noticable in a single drill stem for a short span then I would further question how true the stems are. If the wear is not localized, then I would not suspect a bent drill stem.
I hope this helps,