Среднее время

$time = array (
            '2452:43:44',
            '452:43:44',
            '242:43:44',
            '252:43:44',
            '2:43:44'
        );

$seconds = 0;
foreach($time as $hours) {
    $exp = explode(':', strval($hours));
    $seconds += $exp[0]*60*60 + $exp[1]*60 + $exp[2];
}

$average = $seconds/sizeof( $time );
echo floor($average/3600).':'.floor(($average%3600)/60).':'.($average%3600)%60;