The rough answer is that we have a duty to help to a degree that is reasonable enough that we'll keep doing it. So organizations like Giving What We Can, for ex

The rough answer is that we have a duty to help to a degree that is reasonable enough that we'll keep doing it. So organizations like Giving What We Can, for example, ask members to pledge to donate 10% of their annual income to effective charities, which is a fairly reasonable amount for middle-class people in a developed nation that won't sacrifice much quality of life. Yes, there is always some subtext that a life could've been saved had you given that much extra. But I think it's a mistake to view moral failure as a binary rather than a scalar; every life saved is that much better and we ought to keep that in mind rather than embracing a kind of nihilism that we're not doing everything we can and therefore we should do nothing.