“Weapons aren't indestructible, you know. They have limits! When a weapon's durability reaches 0, that's it: no more weapon. So be sure to keep an eye on those numbers.”
—A Villager in Prologue III chapter in Fire Emblem: Shadow Dragon
The majority of weapons and items in most Fire Emblem games are characterized by their Usage (called WHp in TearRing Saga: Utna Heroes Saga), a preset number of uses that determines the maximum amount of times that a weapon or item is functional before it breaks, thus requiring a replacement. Unlike other RPGs, weapons can only be swung about so many times before they are considered unusable. While there are some notable exceptions, such as the Ragnell, Amiti, and several others, most weapons acquired have around 20-45 uses before they wear out and disappear from a character's inventory. A strategical aspect of playing the games is knowing how to kill enemies with as few attacks as possible in order to save the uses of weapons; money may be very limited, so weapons cannot be replaced on a whim. The durability of most weapons is graded on its strength and availability: the common Iron weapons and Steel weapons have the highest numbers of uses, whereas the superior Killer weapons and Silver weapons have a scant 20 uses, and rare items, such as the Silence Staff and the Bolting Tome have only 3-5 uses.
There are, however, some exceptions to this rule: the starting weapon unique to the Lord character of a specific game is usually a versatile and powerful weapon that has a decent number of uses (some examples include the Mani Katti, the Rapier and the Reginleif).
The rare Hammerne Staff is able to restore full usage to most weapons, but due to its own limited durability, the weapons it is used on should be chosen wisely, as once it is completely used up, the Hammerne staff will break and cannot be repaired. The Armsthrift skill introduced in Fire Emblem Awakening can also help mitigate weapon usage. Missing attacks also does not reduce the durability of weapons, since of course the enemy was not struck. An exception is magic in some games which does use durability when it misses as the spell was still cast.
Counter attacking with a staff doesn't consume usage of the staff.
In Fire Emblem: Genealogy of the Holy War players can get their weapons repaired at castles they own. When a weapon has been fully used it will also become a broken version of that weapon that the player can still use to attack but has a lot less power and accuracy. All weapons, but Earth Sword and some staves have 50 uses in this game, with noted weapons having only 10.
In Fire Emblem Gaiden and Fire Emblem Fates, there is no durability mechanic, allowing players to continuously use the same weapon without having them break. However, Staves in Fates still have a limited number of uses.
In TearRing Saga: Berwick Saga the way durability works is changed. Throwing weapons have a limited number of uses, like in the rest of the series, while magic has MP that slowly regenerates after each chapter. Non-throwing physical weapons have a set 100 durablity that decreases with each use depending on durablity rating (F-7, E-6, D-5, C-4, B-3, A-2, S-1). Each weapon also has a chance to break after every attack depending on its durability (100-61: blue - 0%, 60-41: green - 1%, 40-21: yellow - 2%, 20-1: orange - 4%, 0: Red - 100%). Bows, Crossbows and Ballistae use both Durablity (for weapon itself) and number of uses (for ammunition). Like in Genealogy of the Holy War it's possible to repair weapons for a fee, so long as they don't completely break.