Hi Mark, there is no specific measurement...the cylinder reel serviceability is determined by the amount of blade left above the reel separators (the vertical segments holding the reel together).....when the reel is spun ground until there is almost no 'meat' left, it is then considered to be 'cactus' and ready for the bin.
The serviceability of the 'bottom blade' (bed knife) is usually determined by the engineer (machinist) when the reel and bedknife (still attached to the sole plate) are in the workshop for sharpening....he can fabricate a new bed knife if needs be and has the necessary tools and expertise to remove the screws from the soleplate without breaking it (it's cast-iron).... he will also advise on the reel condition also.
If you can post some pics of the reel, I can give you some idea of its condition.
