I would use them until they are smaller on one end then the other. That be said the last disk blade on the outside is smaller, as to not cut too deep and make a ridge. I"d measure the whole set and then go from there if they are all right round 17 1/2 then sound like they were 18" blades and not too bad of shape. Now if you start at one end with 18" and the other end is 16" you are going to need to replace some soon.
I have an old drag disk that I have gone over 3 times and rebuild the blades many times some of the blades were 12" started with 16" best set I have ever used, old JD model no idea but easy to move light and cuts like it should no weight on top, nice and smooth finish.