When should you stop using basic in-memory caches?

Hey everyone, I'm starting to hit some limits with our current caching approach, which is mostly just in-memory caches inside our services. It's okay for a lot of things, but as our data gets bigger and the load goes up, we're seeing more cache misses and things getting bogged down. I'm curious, how do you know when it's time to move to a separate, external caching system, you know, something like Redis or Memcached? Are there particular performance numbers or patterns you watch out for?