When they get blunt and it takes me several attempts to get blood out (I use a bare lancet, no machine), so usually once a week ?(on 50 tests a week when I'm resting from my cgm).
About once a fortnight, though I do have a libre now so they don't get as much as they used to I found a box of lancets in the cupboard that I got from the pharmacy in 2003 (I've moved house 3 times since then )
When It feels blunt or starts to hurt a bit. I use a Fastclix which holds little drums of six lancets and I still have some of the pack of 200 drums which I bought over six and a half years ago now.
This discussion crops up fairly regularly, so if you want plenty more opinions, you could maybe try a search for those posts too.
About 3 times per week and as I have 3 drums left in one box and an unopened box of drums to use with my FastClicx they are going to last quite some time at that rate.