Hi Michael,
This particular topic is actually a very contentious. Let me state straight out that there are many schools of thought as to how long a disc lasts, but the fact of the matter is that it will eventually fail. No form of storage commercially available is capable of storing your data inevitably.
I've noticed that many people have been discussing and questioning the difference between commercial discs (such as movies and CD's) and discs which you burn at home. Let's tackle this question initially by looking at the difference between the two. It should help explain why these discs are far more likely to last longer than the discs which you burn at home.
How are the discs burnt?
The movies and CD's you buy at shops are created using a completely different process to that of a typical burner. To put it simply, the plastic in the disc is heated to the point where it is malleable and then a metal template is quite literally stamped into that plastic creating tiny grooves which represent the data. The discs are then coated in a very fine layer of metal which acts as a reflective surface on the base of the disc, which the laser in a player can bounce the beam off in order to retrieve the data. It also acts as a protective layer, which prevents the data on the disc from being damaged by scratches. So in summary, the data is quite literally etched into the disc.
Technology like this was not feasible for consumers, so manufacturers started looking for an alternate method of writing data to a disc. The result was the CD burner we all know today. The primary difference between a burner and a commercial disc replicator (the method I described above) is that the data is not etched into the disc, but rather printed. CD burners work by heating a layer of transparent dye inside the disc. The dye responds by darkening (i.e. no longer being transparent). The laser in a burner is so precise that it can burn the dye in the disc in a specific sequence, hence creating the data that you wish to store on the disc.
Why is this a problem?
The problem with this process is that over time, ink starts to breakdown and fade. It's kind of like opening a book that was published 50+ years ago and looking at how much the text has faded on the page as a result of the ink beginning to degrade.
What does all this mean?
Basically it means that any disc you burn will eventually become corrupted and unreadable, as the ink in the disc will inevitably degrade. Even commercial discs will eventually fail as nothing lasts forever.
So how long will a disc last?
The truth is that it depends on a number of factors. Anyone who tries to give you an actual life in years is just speculating. The life of a disc depends on a number of factors. Some of these include the quality of the materials used to create the discs, the quality of the manufacturing process, how well the disc was burnt (i.e. if your CD burner's laser is dirty, then the recording won't be as clean as say a brand new laser), the speed as which the disc was burnt (higher speeds lead to degraded quality in the burning process), the amount of data on the disc as well as how the disc is handled and stored. Things such as heat and moisture will inevitably degrade the life of a disc or oil from finger prints on the disc will also degrade it faster.
In my opinion:
Arbitrarily, I wouldn't trust most discs burnt with a CD/DVD burner beyond 5 years, although the data will more than likely last well beyond that if the disc is stored in a safe manner (more on that soon).
So what can you do?
As I mentioned, there are different qualities when it comes to discs. For example, the ones that you buy in a 50 pack aren't exactly of the highest quality, and can't be expected to last for an extended period of time. A general rule of thumb is the more you pay for the disc, the higher the quality and the longer it is likely to last. If you're looking for long life discs, go down to your local major retailer and ask them what they've got. Many discs which boast a long life tend to be gold plated. They'll probably claim to hold your data in excess of 100 years, but none of these figures have actually been properly tested (I mean, how could they?) Disc manufacturers use all kind of different tests to rate their discs (some of them being quite questionable)and as a result, the life they print realistically isn't worth the paper its printed on.
What do I recommend?
If you're looking to backup precious data, then I'd recommend the following:
* Purchase long life gold plated discs
* Make sure the laser in your burner is clean (you can pick up disc cleaning kits from your local retailer)
* Burn the disc at a slow speed
* Store the disc facing down (not on its side) in a cool dry place (warm and moist = mold). I'd suggest no more than 50% humidity and 20 degrees Celsius (68 deg Fahrenheit).
* If you're going to write on the disc, use a non-solvent based marker.
* And most importantly.... MAKE MORE THAN 1 COPY... Store important data on more than 1 medium if possible (e.g. external backup drive and long life disc)
Hopefully this has enlightened you and has given you some tips on how to safely store your data 
Jaz.