When's the right moment to switch from a simple cache to a distributed one?

We've been using a local in-memory cache, and it's worked well for our current size. But we're expecting a lot more users and more services needing cached data. So, I'm thinking we should probably move to something like Redis or Memcached soon. What signs have you seen that tell you it's time to make that switch? Is it just about handling more reads, or are there other things, like keeping data the same everywhere, making cache invalidation easier, or even just how much work it is to run the system, that make you lean towards a distributed setup?