A related topic to availability is how long the magic in magic items persists. Generally speaking, the less an item can be used, the easier it is to create tends to make sense. If Great Weapons or Rings were common, everyone would have one and Dragaera and Middle-Earth would be very different places. On the other hand, easy availability of limited use Skele-Gro isn’t so bad. On the whole, there are three broad categories of magic item durability:
These are single use items, such as potions, that expend all their magic in one use. Often such items are consumed when used, or disposed of in some other way (ex. the single use Portkeys used for the Quidditch World Cup in Harry Potter and the Goblet of Fire). Cheap, single use, disposable items makes for a strong market for creators and sellers, if they are common. If usage is widespread, then people will be looking for replacements frequently.
These items retain their magic for months or years but eventually the magic wears off. This is the magical equivalent of the home appliance or car and shares a similar place in a magical economy. They are items that might be moderately expensive initially, but can be used for a significant amount of time. Most of the items in Rowling’s world and Pratchett’s Discworld fall into this category.
These devices never lose their magic regardless of how old they are or how often they are used. Obviously, this would not be a good level to build an economy around, as they’d be bad for business. Once they’re sold, there’s no more income to be made off them. But, this level is excellent for rare and exceptionally powerful items such as Excalibur, Glamdring, Spellbreaker, and the One Ring.
In short, how long magic items will continue to work has a profound effect on society and its relation to such devices both economically and socially.
How magical devices are made, and how quickly they can be produced, also has a notable effect on the world. Unique items that take considerable time to make won’t be widespread. Mass produced items can be produced quickly and will be found everywhere, usually.
Rick Riordan’s Camp Half-Blood is home to many, mostly unique, items created by the Hephaestus kids or the gods (and their cyclops minions).
J.K. Rowling’s Magical England has items that are presumably both easy to make and mass produced (those sold in Weasleys’ Wizard Wheezes) alongside handcrafted, labor intensive items (wands & brooms) and unique, powerful object (horcruxes & the Potter family invisibility cloak).
Pratchett’s Discworld also includes both mass produced objects (iconographs, disorganizers) and unique items of great power (Hex).
Unique items that are slow to make become the goals of quests and carry great value. This is true even if their powers are weak—ex. the Staff of Magius in Dragonlance’s Krynn is a rather weak item, but its history and reputation make it desirable. On the other end, mass produced items become common household items that everyone in the setting is likely to have access to—ex. Floo Powder in Rowling.
There is a lot of room in between the ends of the spectrum. If the creator desires a world with widespread items and an item making industry, then mass produced and fast are effectively necessities. A full scale industry cannot be built and produce widespread devices if it takes a month to create one saleable item—they can create a specialty industry catering to select clients, though.
The choice really depends on the desired level of magic use in the setting. The above categories can be used as a way to limit the effects of magic on societies—either the magic system is limited in, or unable to, creation of items or national laws restrict creation (ex. mages need licensing to make items and/or must only do so in certain places to avoid blowing up the neighbors).