It depends on the code. HTML for instance tends to get wrapped around column 80, except where that would break up an entity in an undesirable way. I tend to wrap elisp around column 120 or so. Perl code gets wrapped based on semantics, not column numbers. email is not, strictly speaking, code, but I tend to wrap it around column 70, because although *my* mailreader can correctly rewrap quoted text, some people with whom I correspond use lesser mailreaders. All of this changes in a web forum, where I use the width of the textarea as a guideline.