Cache Invalidation Strategy, When Does It Become Crucial?

Hey everyone, I've been thinking a lot about caching lately, especially for user-generated content that can change frequently. We all know adding a cache is usually a good first step, but at what point does the complexity of invalidation become a bigger problem than the potential performance gains? Is there a tipping point where we should actively plan for invalidation strategies like TTL, write-through, or explicit invalidation based on events, rather than just relying on simple TTL? I'm curious about your experiences and when you've had to move beyond basic caching.