Got through a few plugs (B6HS) when my maggie was packing up. Fortunately, I saved them and started using them again. Tend to use them until a glitch shows up, clean them, put them back - if the problem continues I change the plug, if it doesn't it's 'look elsewhere' time.
On my 1970s two strokes, I ran the plugs until they were thin and pointy - still worked though! Just looked at the ones on the Kwaka and they look fine but crusty round the top - who knows how many miles they've done. On my son's Citigo, change is every 40k, and they still work and look good (kept those also).