I'm on my third winter with them fitted to my winter-hack hybrid, and the wear (to the studs) appears non-existent. I ride commute a 15 mile round trip daily (during the working week) from October through to May (I'm in Edmonton, Canada). From December onwards about half the route is on ice, or ice and snow, with mostly cleared (tarmac) bike lanes for the remainder. I have the tyres with 4 rows of spikes, and it is in no way hyperbole to assert that I could not commute without them.
While I haven't seen any wear, they do get lost. At the start of this winter, I replaced 47 (of 200) that had worked their way out of the rear wheel over the course of the previous two winters. I hadn't really noticed an appreciable deterioration in performance, but last winter the roads weren't too bad. I got the Schwalbe replacement spikes with an insertion tool from
https://www.mec.ca/en/product/5039-709/Steel-Studs-with-Tool-%2850-Pack%29, but they're available online in the UK too. Easy job took about 20 minutes. There were none missing from the front tyre.
Incidentally, I buy winter tyres from the UK (Chain Reaction) - even with shipping and customs duty they are about half the price of buying them here.