How long should an average diamond abrasive last? I have tried several different brands and can not seem to get an average as to how many square feet I can get per set?

There are many factors that contribute too how far a diamond abrasive will go. The type of stone, how abrasive the stone is, how severe the lippage is, the equipment you are using etc.all contribute to the wear of the diamond. In other words, it is almost impossible to calculate the exact square footage a diamond will get. Diamond abrasive technology has improved considerably over the past few years and I have found that diamond life is increasing. Check with the individual manufacturers and ask specific questions. Several manufactures are starting to give guarantees on diamond wear.