CD, DVD life
The Optical Storage Technology Association are the guys to listen to when talking about CD-R life. They reckon UNRECORDED shelf life of 5 to 10 years, while acknowledging some of their members claim anything up to 200 years.
Disc life depends on a pile of things: the quality of the polycarbonate substrate, the quality of the reflective material, the quality of the dye, and for CDs, the quality of the lacquer (CDs are one substrate, reflective layer at the back, lacquer protecting the layer. DVDs are two substrates glued together, with the reflective layer in the middle).
Historically, DVDs have appeared to have shorter lives than CDs. For both types, the "best" reflecting layer were regarded as the Kodak and MAM-A "gold" archival CDs. Kodak dropped out of the market. MAM-A's latest lifetime test results for their Gold Archive product suggest 329 years for CD-R and 116 years for DVD-R.
There are good reasons to select branded disks when making archival recordings: discs can have various types of reflective layer and use around 3 different dyes (depending on how one counts). For best results, each combo needs a different write strategy. These write strategies are programmed into the drive firmware. The drive reads the ID of a blank disc, and selects the appropriate strategy. If it doesn't recognise an ID, it will choose a conservative default, usually causing a slow burn. Some cheap disks use IDs of better-known brands, to ensure the drive uses the appropriate write strategy. Sometimes, the ID is used to deceive the consumer, and the strategy may not actually match the disc construction.
Disc failure modes include porosity of the lacquer/glue and polycarbonate, aging of the glue and lacquer, reaction to the ink used to write on the pen, migration of acid in any paper in the case, outgassing from the case, accelerated aging by light, and being used on any day with a "y" in it.
So, for archival purposes use a brand that your drive will recognise, and use a drive that has consistent write performance on the chosen medium.
Having backed up over the years to almost every technology known to Man, I currently use a RAID server locally and remote hard drives. It's a bit over the top, but drives are cheap. For burning CDs/DVDs, I have cheapo media and a fast cheapo drive, plus archival media and an archival drive.