The technology hasn't been around long enough to really know. Manufacturer's claims of lifetimes of up to 100 years are based on "accelerated aging" testing that may or may not reflect what will really happen. On the other hand, I've seen mention of a Dutch test that found a number of discs were unreadable after 20 months, even some from well-known manufacturers. The safest alternative is to make multiple copies, with discs from various manufacturers, in various formats.
If you make even one copy and keep an eye out for technological obsolescence, you will eliminate most of your risk. Having another copy at a separate location will eliminate almost all of the remaining risk. Everything else is aimed at eliminating the tiny residual of happenstance. Most people that lose data hadn't taken the first step.
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