I have introduced the idea that all magic even permanent magic breaks down over time. As a child of 1st and 2nd edition Greyhawk I have read many stories detailing the breakdown of magic prisons and the like? I have not quantified how quickly the breakdown occurs but I think it is obvious that it depends on the level it is cast at. Has anyone ever experimented with this idea?
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That sounds as if it would change the magic in the world pretty drastically from the default. If breakdown is a monotonic function of power, your world should have a substantial shortage of either high-level (if high concentrations of magic are unsustainable) or low-level (if low concentrations aren't stable) magic.
In the first case, you might find it difficult to explain the existence of ancient and powerful magics. In the latter, the existence of minor magics is hard to justify. This is not to say such justification is impossible, but you'll want to think about it before your players do.
Alternatively, you might want to consider the possibility of adding reduced decay rate as a cost enhancer when creating magic items. For instance, reducing decay rate by 50% might be a +1 enhancement to a sword, while 90% might be a +2. (Numbers chosen nearly randomly.)
If you have fewer magic items, you'll need to adjust the ratio of EL to mean group ECL to maintain encounter balance. This will be especially true for high-CR enemies. (The adjustment is possibly non-linear, so this may be tricky.) If you plan to have about the same number of items as a more typical campaign, you'll need to drop the cost of creation to compensate for increased wastage. This is probably more straightforward, but may not give the result that you are looking for.
So long as you think through the implications, this should work just fine. Make sure your players are OK with it before you implement, though.
When I wrote this I was not considering magic weapons only magical effects. I see how I wrote my post that it would be taken to affect magic items. I suppose something in the binding magic to a physical item prevents decay, but magical effects such as guards and wards or sigils and glyphs slowly loose their effectivenes. So having cleared up my faux pas, what do you think?
I agree with ericthecleric. The breakdown of permanent magical effects in the game of D&D is more of a plot device used in adventures than anything else. It assumes that some malign force is struggling against the magic slowly weakening it (such as a horde of demons pushing against a permanent magical ward surrounding a city). Of course, in fantasy literature permanent magic sometimes erodes over time. This usually takes centuries or thousands of years, so has no real practical game application (except again as a plot device, such as the invisible tomb that suddenly begins to appear as the magic that made it invisible fails).
You could make up a rule for permanent magical effects. For instance, a permanent spell might have a 1% cumulative chance per year minus 100 times the spell level of vanishing. Therefore a permanent 1st-level spell would last at least 100 years before breaking down, and would break down sometime in the 101-200 year period, while a 9th-level spell would last 900 years and break down sometime in the 901-1,000 time period.
Hmmm... yes... influence of moons and stars as another DM excuse for arbitrary happenings in the world.... I'll take it! :)