Manual:$wgActiveUserDays

Statistics and content analysis: $wgActiveUserDays
The number of days within which a person must make edits to be considered an "active" user
Introduced in version:	1.17.0 (r69495)
Removed in version:	Still in use
Allowed values:	(Positive integer)
Default value:	30
Other settings: Alphabetical | By function

Notes

The number of days within which a person must make edits to be considered an "active" user. In other words, if no edits are made by the user in this amount of time, the user is considered inactive. Will affect the number shown on Special:Statistics, Special:ActiveUsers and the {{NUMBEROFACTIVEUSERS}} magic word in wiki text.

You might want to leave this as the default value, to provide comparable numbers between different wikis.

Note that the initSiteStats.php maintenance script should be run (with the --active parameter) periodically in order to actually update this statistic. To be strictly accurate, it should be run once a day, but for many wikis it's enough to run it weekly.

Old version

A variable with the same name was present from version 1.13.0 to 1.15.x. The description was:

This, along with $wgActiveUserEditCount define what User::isActiveEditor() considers to be an active editor. Simply: if a user makes at least X edits in at least Y days, the user is considered active. By default, it's set to 30/30, so 1 edit/day.

This was never actually used.