It's unlikely to take days. A few months ago they replaced a broken key cap for me in about 20 minutes.
https://www.youtube.com/watch?v=4KuVvb9DTaU (from about 5:20) talks a bit about what's involved in an actual keyboard replacement, if you ignore some of the histrionics.
I think from what I've heard that for actual replacements, Apple will swap out the whole 'top case' subassembly, which could be a 4-500$ part if it's not covered under warranty.