Database Sharding: When Do You Really Need It?

hey everyone, i've been thinking a lot about scaling databases lately. we've been pretty lucky and haven't hit massive scale yet, but it's definitely on my mind. we've done read replicas and some optimization, but i'm curious about when you all actually start looking at sharding. what are the main signs that your current setup isn't going to cut it and sharding is the next move? any horror stories or success stories from when you implemented it