日数については、2日ということで、自己解決しました。
/wp-includes/ms-functions.php
488〜508行目
// Has someone already signed up for this username?
$signup = $wpdb->get_row( $wpdb->prepare("SELECT * FROM $wpdb->signups WHERE user_login = %s", $user_name) );
if ( $signup != null ) {
$registered_at = mysql2date('U', $signup->registered);
$now = current_time( 'timestamp', true );
$diff = $now - $registered_at;
// If registered more than two days ago, cancel registration and let this signup go through.
if ( $diff > 2 * DAY_IN_SECONDS )
$wpdb->delete( $wpdb->signups, array( 'user_login' => $user_name ) );
else
$errors->add('user_name', __('That username is currently reserved but may be available in a couple of days.'));
}
$signup = $wpdb->get_row( $wpdb->prepare("SELECT * FROM $wpdb->signups WHERE user_email = %s", $user_email) );
if ( $signup != null ) {
$diff = current_time( 'timestamp', true ) - mysql2date('U', $signup->registered);
// If registered more than two days ago, cancel registration and let this signup go through.
if ( $diff > 2 * DAY_IN_SECONDS )
$wpdb->delete( $wpdb->signups, array( 'user_email' => $user_email ) );
else
$errors->add('user_email', __('That email address has already been used. Please check your inbox for an activation email. It will become available in a couple of days if you do nothing.'));
}