I would have thought it would be less than 15 year to be honest.
The DVDs i have of important data will be dumped in a couple of year and replaced with either BluRay or HD-DVD disks. That way i can have much fewer disks to deal with.
Also if you are woried about disks deteriorating, you can always invest in medical/archival quality DVD-Rs. I don't know how much they are as you can't really get them through normal retailers. But they are supposed to be much more reliable in the long term.