Hmm, once or twice per feed I believe, when they're young. I only have one snake so not a great deal of experience, but I would have expected my boy to have gone by now if he ate Saturday. That having been said I wouldn't be too worried yet; just keep an eye out and hopefully he'll go soon.
Hope that helps; others with more experience will probably be able to give you a more definitive answer :P
Most people use aspen shavings, but there's nothing wrong with wood chips from the snake's point of view, just means it's harder for you to see when he 'goes' :)