use the PHP time() function to compare and divide the result by 3600 to get the difference in hours
http://www.php.net/manual/en/function.time.php
<?php foreach($tutores as $t) {
echo floor((time() - $t['lastlogin']) / 3600) . ',';
} ?>
EDIT: Updated to use floor() in example. Could also use round() or anything else to format the result as you want