SHOW GLOBAL STATUS LIKE 'Uptime';
Result
+---------------+--------+
| Variable_name | Value |
+---------------+--------+
| Uptime | 322477 |
+---------------+--------+
1 row in set (0.00 sec)
(The uptime is given in seconds)
More status variables
There are many more status variables available.
Show them all with this query:
SHOW GLOBAL STATUS;