The Mormon Church says, "tithe". Loosely paraphrased, they say, "10% is a lot, and 10% is enough." This is actually very smart, because they've created a difficult but achievable standard, a way to be a member of good standing in their tribe.
When my dad ran the local United Way drive as a volunteer, he pushed for one percent. "One percent isn't a lot, but it's enough."
The model here is duty. How much do I owe? How much meets my obligation? What is my fair share? tithing and taxes have that shape. What does it take to make a difference for the causes I care about? How much can I afford? Those are another way of addressing how much is enough.
