It's not possible to enter and save a number larger than 7 for User preferences / recent changes / Days to show in recent changes. If one tries the entry reverts to 7 after pressing the save button.
I believe it's because of this line:
$wgUser->setOption( 'rcdays', $this->validateInt($this->mRecentDays, 1, 7 ) );
The value should be increased (at least up to 30 in order to reflect the possibilities in Special:Recentchanges), and/or the message [[MediaWiki:Recentchangesdays]] could be changed to reflect the restricted range of choices.
If only certain numbers are to be allowed (as in Special:Recentchanges), it should be changed to a drop-down box rather than a write-in field.