[quote=Milieu] So do all materials break down over time? And depending on the power level of the spell (especially if its a powerful spell) wouldn't that do more damage to the material itself?[/quote]
No, not all materials break down over time. The magic does not naturally damage the vessel (unless it's the desired effect..). 
Compare glyphs that are: 1. Carved on a wooden bowl 2. Engraved on a brass pot 3. Cut on a diamond palette. The instructions will be [collect water from air, fill vessel].
When activated, water will slowly collect on every vessel and you drink the water. For weeks you use the vessels activating it every day and drinking from it. A month later, you notice the water in the bowl takes a very long time to fill and it does not fill to the brim(looks dodgy too). 2 months it stops working altogether. Same thing with the pot but it takes 8 years for the magic to wear out the integrity of the pot. The diamond palette however seems to last indefinitely as at least 50 years wouldve passed before it is completely useless, assuming daily usage.
You could say that an object with a higher aesthetic value will have higher integrity. The act of engraving also increases the integrity which is why very well crafted glyphs work better than ugly looking ones. Magic is very picky. 
About the dagger, because the nature of the magic is to produce light, instructions: [shine], it would not damage the material. iron dagger will last you a few days, steel probably months-years(depending on the grade), Mithril decades, diamond(?) a very long time. Alternatively, materials could be damaged if instructions were [pierce and deform metal] and you used an bronze dagger and tried jabbing at a hardened steel block. Scenario one, glyph is dormant, jabbing the block makes you look like a fool but nothing happens out of the ordinary, maybe the dagger is blunted but the block has hardly a nick on it. Scenario 2, glyph activated, the magic will insist on piercing but because the bronze is shyt it will shatter, and the user might die, regardless of how magically potent the user is. Really depends on the nature of the magic.

@Ichthys: Glyphs magic, when used offensively is dangerous to the user as seen above. Its also quite useless in direct combat unless you have top of the range equipment, which Flowryde doesnt own, but he does makes some for the noble people of D'Cerfs. Flowryde is a broke ass kid. Since instructions have to be precise, using it in battle is inefficient. You want to use magic to create invisible barriers? No probs, leme get my staff out and draw a bunch of glyphs. Hey please stop hitting me, im concentrating on drawing glyphs, also, can you stand in between those two lines, the magic wont work anywhere else. 
He does have a battle outfit that has glyphs weave in to prevent burns and grazes, but its only as effective as the next persons chainmail. Flowryde is has high magical potential but physical limits (money,time) really constrains his magic.
Flowryde carries 2 runes, one that says [make heat] and another that says [collect water]. quite useless in battle. But it can heal cuts simply by gathering blood in the wounds and burning them close. You can trust me with the limits, unbalanced characters are no fun ;P