When do ya really need database sharding

hey folks, been thinking about scaling databases lately. we're hitting some limits with a single large relational instance, and read replicas arent cutting it anymore. ive been reading up on sharding, but it seems like a big jump in complexity. what are the key indicators that tell you its time to seriously consider sharding? is it purely based on query performance, storage size, or are there other operational factors to watch out for? would love to hear about your experiences, especially the pain points when you decided to shard.