I am going to try and combine my two trips onto 1 PhotoPass CD. I had the same question and I wrote to the PhotoPass site. Here is the response that I got:
Hello Michelle,
Thank you for your request and interest in Disney's PhotoPass CD. Photos taken within 90 days can be included on your Disney's PhotoPass CD.
Please note that the expiration for DisneyPhotoPass.com does have a strict 30 day policy, but we allow guests to purchase a Disney's PhotoPass CD for the life of your photos (a total of 90 days from when they are first taken).
Please let us know if you have any additional questions or concerns.
Sincerely,
Lynne Murray
Disney's Imaging Quality Assurance Team
Original Message Follows:
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Hi! We're going to be taking two WDW trips in the near future - one at Halloween and one at Christmas. I wanted to know if I keep the PhotoPass card that I get from my first trip in October, and use it again in December, will I be able to get all of the photos from both trips on one CD? I guess the simple question is, how long do the pictures stay in the PhotoPass system?
Thanks!
-Michelle