When should you break things down into microservices? That's the big question, isn't it?

so, we've had this monolith for like 5 years, and it's really showing its age. deployments are slow. it's tough to scale certain parts without affecting others, and getting new people up to speed is just awful. we're thinking about microservices, but wow, the initial cost and how complicated it all seems is a lot. has anyone else done this? how did you know it was finally time to split things up, and what totally surprised you or what do you wish you'd done differently when you made the change