If you make the reservation over the phone it should show up on DVCMember.com immediately. Email confirmation is usually sent out same-day, but bear in mind that any email can get grabbed by spam blockers in your software, your ISP, etc.
If you submit an electronic reservation request via the website, those are glorified email messages which are processed on an inconsistent basis. They say to allow up to 48 hours for those requests to be processed.