So, when should you actually add a database read replica?

hey everyone, ive been wrestling with database load lately. we've got a single primary that's starting to feel the strain from all the reads. i know read replicas are a common solution, but im curious about the tipping point. at what point did you decide it was time to add a read replica? what specific metrics or load patterns signaled that it was the right move? any gotchas or best practices to share from your experience when setting them up or managing them?