There is an argument commonly known as the Drowning Child Argument, which goes something as follows:
Imagine that you’re walking across a shallow pond and you notice that a small child has fallen in, and is in danger of drowning…Of course, you think you must rush in to save the child. Then you remember that you’re wearing your favorite, quite expensive, pair of shoes and they’ll get ruined if you rush into the pond. Is that a reason for not saving the child? I’m sure you’ll say no it isn’t, you just can’t compare the life of a child to the cost of a pair of shoes, no matter how expensive.
The problem with this argument is that the current best estimate of the cost to save a life is about $3500. I do not think that most people own shoes that cost $3500. Even their favorite most expensive pair of shoes probably isn’t $3500; it might be a tenth of that. However, the problem with specifying the cost of the shoes in the analogy is that most people will assume the person is rich, and therefore can trivially afford to buy another pair of absurdly expensive shoes.
A more correct analogy is something like this:
Imagine we live in a fantasy world in which children are sometimes teleported away from their homes to drown in ponds. However, it is possible to go on a quest to save the children. The average person can complete one quest in about five weeks. However, some people are very skilled, and can complete a quest in a week or even a day; others are less capable, and may take months to finish the quest.
(Also some people argue that instead of focusing on the teleporting drowning children, one should help spread norms of not slaughtering orcs or work on preventing evil wizards from destroying the world.)
My intuitions for this world suggest that it is not actually mandatory to spend all of your time going on quests, unless you happen to be extraordinarily good at quests, and a person who completes one quest a year has probably discharged their duties with regards to the teleporting baby epidemic and can spend the rest of their time farming dirt or counting how many lice they have or whatever people do in medieval fantasy settings.
Moving that back into this world, the teleporting drowning child argument suggests that one should donate about ten percent of one’s income, unless one happens to be wealthy in which case one should donate more.