So, when should you switch from vertical to horizontal scaling?

Hey everyone, I'm trying to nail down the decision point for when to move from scaling up, you know, vertical scaling, to scaling out, which is horizontal scaling. Our main app is on one big server right now and it's really starting to struggle. We could just add more RAM and CPU, but I'm wondering what signs or metrics you all look at to decide, 'Okay, it's time to add more machines instead of just getting a bigger one.'