It is a site-wide setting. It's currently set to 30 and I have not purposefully changed it. I can check to see if the default value changed during the update, but I don't believe it did.
There is an extension that allows it to be set per-user, but it hasn't been updated for the latest version of PHPBB. There are also performance implications as that number goes up and it's probably better to keep it at the same value for everyone for caching reasons.