when do you break out a new service from an existing monolith

So, we have this fairly large monolith that's been working great for us. But lately, a few parts of it are growing super fast and getting pretty complicated. It seems like a good spot to break out into a microservice, but I'm not sure when to actually do it. What do you guys look for to know it's time to split something off versus just leaving it in the main application? Are there any particular numbers or warning signs that tell you, 'Okay, this needs to be its own thing now'?