Events that are enriched off of other data in the same DB.
The system design predates me, but it is solid (albeit difficult to operate at this scale - usual stuff like schema changes, replication bootstrapping).
We keep hot masters (typical MySQL master-master-slave(s) setup) on standby.
Bootstrapping one from scratch (like if we need to stand up a new replica)? We restore a disk image on GCP from point in time snapshots, then let it catch up on replication. So it'll depend on how far behind it is when it comes online.
TLDR: a few hours in the worst case. ~Zero downtime in practice.