I'm not a shop owner but I do a lot of small engine work from my house when I can... I've fixed 6 push mowers for neighbors so far this season, and repaired several trash picks and a tractor (thanks to this forum might I add! Appreciate it!) I got 4 leaf blowers, worked on some string trimmers, have a go kart, a chain saw, a couple generators, a couple of pressure washers, yard vacuums, and yet...
I've never experienced a bad spark plug. I really am dumbfounded. I've worked on probably 40 or so engines at this point, and never encountered a non-firing plug. the NGK ones I have all work fine, so do champions, etc.
I'm Sure this will get me a lot of hate but admittedly I don't change plugs much anymore, because like I said they just aren't ever goin bad for me. If I have a rough/bad running engine or no spark I'll put a different plug in as a test measure to see if it improves, but otherwise, I just leave them. So far, not one is bad!