Depends on how often you plan on visiting. If the upcoming trip is the only one you will take, and you want to have the actual card with you, you should probably order it 4-5 weeks ahead of your visit. There is some lead time to actually get the card.
OTOH, if you are going to visit several times, you may want to wait until the last minute and get the email/fax confirmation. I think you have to actually pick that up at one of the ticket booths on-site. They used to email them to visitors, but I think they stopped because of abuse.
The reason you may want to wait is the card is actually issued for about 13 months, so you can squeeze one more trip in on the DDE!
Keep in mind that you have to be either a Florida resident ($75) or an Annual Pass holder ($50) to purchase the DDE. DVC members are not eligible for the DDE by virtue of our membership alone.