There is currently no way to find the MariaDB cluster's uptime. You can find individual hardware node's uptime, you can find the database uptime for each node e.g. with SHOW GLOBAL STATUS LIKE 'Uptime', but not for the cluster database as a whole.

(and longest uptime of all the nodes would not necessarily give an approximation either as nodes can come and go, but the DB should still be up)

Suggest there should be a wsrep_uptime ?