Hey there Tempus ,
The lifespan of a cd-recordable is relatively short , because it is affected by Ulra Violet radiation , the acids in your hands , environment and various other minor things.
Some (Dutch) research showed , that if there is not enough protection , the data gets deteriorated within a couple of years.
Especially for the ultra-cheap recordables. There have been lots of troubles with low quality recordables as well. (I do believe CMC magnetics is one of them).
DVD Recordables have fairly the same process ; Crystals get heated and form a 1 or a 0 (Not entirely true , but let’s assume for the sake of argument).
So , if there is little protection , there is lots of deterioration. I’ve already got a DVD+RW , which i can’t format anymore.
I have no idea what the average lifespan of a DVD is though , beause i don’t know which manufacturers make them and if there are quality tests.
But if you keep your DVD’s out of (sun)light and humidyty , you can have quite a longer lifespan than an average VHS tape. I think 5 to 10 years minimum.