You can't do that directly with PHP. The best you can do is to record when the cookie was created (e.g. in a database) and then compare that value to now and find the difference. Subtract the difference from what you set the cookie to and you'll have how long the cookie has left.
example
// get when the cookie was set. here we record that time to a database
$query = "SELECT lastvisit FROM users WHERE userid='$userid'";
$result = mysql_query($query) or die(mysql_error());
list($lastvisit) = mysql_fetch_array($result);
// do some math
$lastvisit_timestamp = strtodate($lastvisit);
// the +3 Month was what we used when we set the cookie
$expiration_date = strtotime("+3 Month",$lastvisit_timestamp);
$time_left = time() - strtotime($lastvisit);
If $time_left is negative then the cookie has already expired. If it is positive then it is the number of seconds that the cookie has left
