I didn't find this question or a related answer to my question.
Some spells like "Avada Kedavra" are spells that are intended to do one thing, the conjurer focuses on that spell and when they stop focusing, or the blast doesn't hit it's target, or the wizard dies, the spell ends.
A spell that opens a locked door for is another example, it either fails immediately or the door unlocks, either way the spell is over.
However, what about spells like the charming spells Harry, Hermione, and Ron used to conceal themselves?
They would get to a camp site, one of them would walk around the perimeter of the camp site and cast spells to conceal their location. Does the spell last in perpetuity? Does it only last while the conjurer is focusing on it? Does it last until it is dispelled by a counter spell? Does it just run out eventually?
I don'think the conjurer has to focus on it. It always appeared in the books and the movie that once these spells are cast the conjurer doesn't think about it again.
The charm spells are just an example. A love spell would fall into the same category as well. I believe the love spell Voldemorts mom put on his dad lasted until her mom released it and she didn't have to focus on it. There are many other examples of these types of spells as well.
You might want to consider Lily Potter's spell on the fish she gave to the professor as a gift in your answer. I believe it lasted until she died.
I don't know if each spell has a distinct amount of time that it lasts or if spells of this category follow a general rule.
Author's notes, interviews, etc. are welcome.