How do I truncate single posts?

While alchymyth makes a good point, because single posts is supposed to be for a full view…you could add this to your functions.php file:

function limit_text($text, $limit) {
	if (strlen($text) > $limit) {
		$words = str_word_count($text, 2);
		$pos = array_keys($words);
		$text = substr($text, 0, $pos[$limit]) . '...';
	}

	return $text;
}

And then run your content through through that function like so:
limit_text(the_content(), 5); // 5 being the amount of words you'd want to have before truncation.

However, there are better ways to do this, including allowing the user to truncate normally and calling the_excerpt().

You can see more on how to do these things here: http://wordpress.org/support/topic/truncating-the-content-output?replies=5