for me and my soil type,     bearing condition usually dictates  when the  disc is  retired.      The rare bearing failure  when the disc is  close to full size gets new bearings..  I've  run them down to  11- 12 inches  on my 5400,  (14 is new)  then when the bearing goes out... they are done.   
   but  have to   adjust  the depth gauge wheel (packer wheel)    on those rows to maintain  even depth.   
Each year,  I'll   put about 5- 6 sets of new discs on,     and put brg's in 2-3  pair.