when should i start thinking about caching

hey everyone, i'm still pretty new to system design and trying to get a handle on the common patterns. i know caching is a big one, but i'm not sure when it's actually useful. like, is it just for really high-traffic sites, or are there signs to look for sooner? say, if my database queries start taking longer, or if i'm seeing the same data requested over and over? what makes you decide it's time to add a caching layer