odd question...
try using time() at the top of your script then print it at the end e.g.
<?php
/can't be bothered with the PHP tags, if this confuses you then you shouldn't be a programmer/
$timer = time();
//load of crap that no-one really cares about
$took = time()-$timer;
print("This following script took about".$took." to execute.");
?>
its goes something like that.