If I get dvd quality score between 90 and 99, how many years can that media keep my datas ?? any idea ??
Let me look into my crystal ball…
Longevity of optical media depends on many factors, the (initial) burn quality being one of them. Some dyes ‘age’ faster than others, depending on the climatic conditions. The mechanical quality / robustness of optical media varies as well. It’s therefore impossible to say how long your discs will last.
Just stick to quality media, treat you discs well, and your data should be safe for years to come. I have CD media that I burned almost 10 years ago, and the readability ist still excellent, because I paid more for quality media. The money was well worth it, as can be seen here, for example.
Quality scores have no realtion to disc life
I have some really cheap discs that were orignaly 1x DVD-r discs. When I burn them I get Q scores of 97ish.
I also have some Verbatim 16x +R’s and they give me Q scores of 92-93 as they are a bad batch.
1 month later and the cheap DVD-R discs cannot be fully read back, POF errors at speed and high PIF errors, the DVD+R’s still have the same error rates though
Good quality media is needed for storage, if you dont mind the data having to be re-copied later (e.g. a DVD backup) then you can get away with slightly cheaper discs (not the mega cheap ones though)
thanx qwakrz and packetloss with crystal ball 
well, then how can I learn the dyes of medias??
note: a study for dyes
http://www.itl.nist.gov/div895/gipwg/StabilityStudy.pdf
Even with a QS of 90+ there is no guaranty your media is readable right after the burn, not to tell in a year or two.
I’ve had 91-92 QS 16x burns that won’t even pass a simple transfer rate test, not to mention read back to HDD…
Always take QS’s with “a big grain of salt” and better perform a transfer rate test before settle back and wait till your media deteriorates. 
So a good transfer rate test can indicate whether the disc would live for a long time or degrade too fast ? or what ?
What realy would guarantee that our data is safe for years to come "if we made sure that we handle it with extreme care and keep them safe " ?
A bad transfer tests indicates that the drive used to perform the test has problems reading the disc. This might mean that other drives could also have problems.
A good transfer test indicates that the current drive, and probably other drives of the same model, can read the entire disc without having to re-read anything. It doesn’t guarantee that the disc can be read in other drives, but it increases the likelihood of that.
What would really guarantee that a disc can be read for years to come?
That one is easy: Nothing, I repeat nothing, will guarantee that!
Using quality media and a burner capable of burning with good initial quality improves the chances. Storing the media properly also improves the chances. Burning the data onto multiple media of different origin also improves the chances. Scanning your media to detect early deterioration may increase the chances, but OTOH every time you handle a disc there is a small chance of damaging it.
They also make media now that has coating on it to supposedly protect it from decay. Have not tried any though so not sure if it is worth the extra price.
Does media degradation applies to both DVDs and CDs ? I ask that because I have CDs from 8 years ago and they still read fine .
If dvd media degrade more quickly than cd media , I will use cds from now on 
me too
I wonder what would Blu Ray or HD-DVD do then , the disk may take a week to get unreadable :a