For the time being, I prefer 10 minutes so that there are no subsequent edits. They might change posts to which replies have already been made without having record how the original post looked like. More posts in a row from the same poster within 20-30 minutes is still better than replies to posts where wording is no more what was replied to.
BTW, you may find other useful tips in as well (like no external links) - they are, of course, linked to from Board Guidelines.