DVDs whether single-layer or dual-layer, use a dye to record the data (M disks being the exception). Dyes are naturally unstable, and most any dye will fade over time.
Archivists categorize dye fading in two ways - dark fading, and light fading. Dark fading occurs when a disk is in complete darkness and is primarily due to molecular decomposition from ambient heat. It can be slowed by keeping the disk cool (I refrigerate my important, long-term data disks). Obviously, putting your disks in a hot area (attic, cars, etc.) will greatly accelerate this type of fading.
Light fade is attributable to fading caused by exposure to light. Exposure to the upper end of the spectrum (blue, violet, and UV) can cause significant fading over time. Keeping your disks in an opaque case will prevent light fading almost completely. Never lay them out in the sun where they would get a dose of harmful rays. Playing your disks will contribute insignificantly.
All that said, your disks should last a long time if well managed. It's quite possible that we will move on to the next generation of storage (whatever that is) by the time your disks expire (if properly kept), and you won't be able to find a player to play them on (unless you've stored one or have one available).
So, as the previous posts have said, your disks may last a long time, and keeping them in a friendly environment is important, but the actual lifetime is impossible to predict.
By the way, other failure modes include surface damage, de-lamination, and reflective layer deterioration, but these are less common and less likely to occur compared to dye fading (well, maybe not surface damage). M disks are touting centuries of lifespan (take that with a grain of salt), so if you really want a long term backup, make it on an M disk.