when do you start thinking about a database read replica?

Hey everyone, I'm running into some scaling problems with a pretty busy web app. The main database is starting to struggle with all the read requests, though writes are still fine. I know about read replicas, but I'm wondering when it actually makes sense to add that complexity and deal with replication lag, just for better performance. What have you all experienced? What do you look at to know it's time to set up read replicas?