Wikimedia Bugzilla is closed!

In SpecialPreferences.php, line 850:

http://svn.wikimedia.org/viewvc/mediawiki/trunk/phase3/includes/Preferences.php?revision=101358&view=markup

$defaultPreferences['watchlistdays'] = array(
                        'type' => 'float',
                        'min' => 0,
                        'max' => 7,
                        'section' => 'watchlist/displaywatchlist',
                        'help' => $context->msg( 'prefs-watchlist-days-max' )->escaped(),
                        'label-message' => 'prefs-watchlist-days',
                );

This is a hard coded limitation that doesn't properly reflect the possible range supported by Special:Watchlist (that is, all changes made that are still present in the recent changes table).  The best fix is to have the 'max' parameter calculated using '$wgRCMaxAge / 86400'.

If it still needs to be hard coded, set it to either 30 (Wikipeida) or 91 (default) instead.