when do you break up a monolith into microservices, tho

i'm looking at this pretty big, old application, and yeah, it's showing its age. it's getting hard to scale, and we're just not building new stuff as fast as we used to. i've thought about microservices, but that seems like a huge leap, you know? what things do you usually watch for that tell you it's time to start breaking up a monolith? is it the team size, specific performance issues, or something else completely