During upgrade from 1.8.3 to 1.11.1 I've put maintenance_mode = 1 in my config.ini.php.
But during update this line was removed probably when adding trusted_hosts line.
My DB update took 2h but was successfull (good job guys :)
This is by design (maintenance automatically turned off after successful upgrade)
Ok, but this was during upgrade!
I saw something like - upgrade from DB 1.9-b9 to 1.11, so this was some step in upgrade process.
And it was in about half way of the upgrade when when we consider time (after 1h more or less of 2 hour process)