When should you add a caching layer?

hey everyone, been thinking a lot about performance lately. we all know caching can really speed things up, but i'm curious about the exact tipping point. is it when the app starts feeling sluggish, or when specific database queries are eating up all the cpu? do you usually wait for user complaints, or have some monitoring metrics you look out for? what are your go-to strategies for deciding when to add caching, and what kind of caching do you typically start with, like in-memory, redis, or memcached?