Weapon durability is stored in a single byte, so it can't go over 255 base uses. Even then, in PoR, RD, the GBA titles, and Genealogy of the Holy War (and possibly more, but I haven't tried), for some reason, the game cannot permanently store any usage level over ~80. So, you could give an iron sword 200 uses, but even if you don't use it at all, when you load the next chapter, it will have significantly less, generally ~60 if you give it 255 uses.