Manual:$wgRCMaxAge

Details
This setting holds the length of time that recent changes history should be retained for (in seconds).

Whenever anything happens on the wiki (e.g. page edit, move, delete, new user, etc.) a row is added to the recentchanges table giving details about the change. Periodically any entries that are older than the length of time given by this setting are removed from the table, and therefore from the list of recent changes.

What this means is that changing this value won't suddenly allow you to see more entries on the recent changes page, as the changes have already been removed from the table, however any entries that are still in the table will not expire until the new limit is reached.

If you want to re-populate the recentchanges table having changed this setting, then you need to run.

Affected pages
This parameter affects recent changes, related changes, watchlists, checkuser, and list of new pages, but not history and user contributions pages.