2 Years of Service
From what I've read, it's about different quality in different cd/dvd's. If one bought the cheapest writeable ones, they could last as little as 5-6 years.It's been years since I tried a CD or DVD, so I don't know how mine are. I didn't even know of this!
But I remember that with scratching most DVDs that weren't read by their respective devices could still be copied with a computer because it didn't need to be read in real time.
This saved me lots of times during the PS2 era.
From an archival POV, I've heard the standard is spinning hard-drive or high capacity tape, always having copies on another drive/media, and a schedule for migrating onto new drive/media often enough that risk of dataloss becomes neglible....