The abrasive in MM wears and clogs, just like any other abrasive. It is washable, so can be refreshed that way, but the wear-out factor is still there. You are your own best gauge. Compare a used piece to a new. Major difference! I have been making a living at woodworking for decades, and have a great disdain for spending more time than is necessary when sanding. I go fresh on the abrasives just as soon as I notice any significant fall off in performance. There are those here on the forum that are of the opinion that MM ALMOST lasts forever. I personally think they just like to spend their time sanding a lot more than I do.
I agree with Steve. I am not sure how many pens I do get fininshed with a set ,but know as soon as I see that there is a difference in the time spent sanding I get new.
MM is my polishing cloths. As long as I have the ridges and shiny spot sanded out I switch to MM. Also wet with a little bit o liquid dishsoap added
I dont like to use it dry of just the wood either. 600 sanding is high enough before appling CA. If you get it to smooth the CA has nothing to hold onto.
Works for me. As another member always says Thats my two cents worth.