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.