The lifetimes of carefully stored CD-R dics (in the dark, low humidity, moderate temperatures, protected from dust, dirt and moisture) is estimated to be anywhere from 20 to 100+ years, based on accelerated aging tests. In general discs made from more light sensitive dyes like cyanine fall at the lower end of the range, followed by Super AZO and AZO discs and finally phthalocyanine discs, due to phthalocyanine's natural relative resistance to light, at the upper end of the range. Of course, these claims are based on projected lifetimes rather than actual measured lifetimes. The projections are made based on accepted scientific principles, but unexpected variances can not be ruled out with 100% certainty. And lifetimes will vary according to the care with which the discs are manufactured in the first place, making discs from a reputable manufacturer a better choice for long life.
Mitsui discs are made with phthalocyanine dye and are supposed to have good archival characteristics, so you should expect a long lifetime from your discs. At one time Mitsui claimed a 200-year lifetime for their archival grade discs. You can read what Mitsui is currently claiming regarding their discs here:
For maximum archival lifetime, a disc with a true gold reflective layer like the Mitsui Gold discs or the erstwhile Kodak Gold discs should be used.
For extra insurance, you should regularly test your archived discs for degradation, and copy to new discs if signs of trouble show up.
In the end, good quality discs burned on a good quality drive and handled with proper care and monitoring are likely to outlive the technology as a whole. In other words, the discs will be probably still be good when you can no longer buy readers for them and have long since transferred their contents to newer technologies.