I've gone all technical and actually bought myself a chain checker a while back. Now do you change the chain at 0.75 wear or 1.0? I don't want to end up in my old ways where I'd wait until chain, cassette and probably mech were jumping all over the shop!
I run xt cassettes and sram 990 chains. No significant signs of wear on cassette (I use two rear wheels depending on situation and use the same chain on both cassettes with no issues), and no wear on the front rings.

0.75 if you don't want to replace the casettes

Cheers

as above, catch it between the two and you should get to keep your cassette.

hijack… how many chains to a cassette would you get doing this?

I usually 3 per cassette & 6 per front ring

'Tis done, going for a blast in a minute to make sure everything is happy!