The question of "how much giving is enough?" has always dogged the wealthy. John D. Rockefeller, one of the fathers of American philanthropy, tithed 10 percent of his income to his church ever since his first paycheck.
Warren Buffett and Bill Gates took the 10 percent even higher, arguing through their Giving Pledge organization that the rich should give away at least half their wealth.
There is, of course, no "correct" level of generosity for the rich. But a new study from Bank of America gives a broad benchmark. It found that American millionaires give nine percent of their income to charity. That level has remained constant between 2009 and 2011.
Total dollars given by the wealthy fell over the same period, however, as top incomes fell and the number of rich donors dropped. The study found that the average dollar amount given fell to $52,770 from $56,621. (Read more: Affluent Plan to Spend Less If Obama Wins)
The study found that 95 percent of millionaires gave to charity in 2011 — down from 98 percent in 2009. Fully 89 percent of millionaires gave their time to charities (as well as their money) in 2011, with more than half of the millionaires giving more than 100 hours of their time.
More than three quarters of the wealthy plan to give as much or more over the next three to five years. Even if the charitable deduction is removed from the tax code (as non-profits fear) more than half of millionaires say their giving will no be effected.
Just as important as the giving amounts, however, are the causes and strategies supported by today's philanthropists. (Read more: Do Tax Crackdowns on Wealthy Pay Off?)
The study said the wealthy give most when they can be personally involved in a philanthropy.
"During the past decade, we have seen donors become increasingly impact-driven and strategic in their charitable activities," said Una Osili, professor of economics and philanthropic studies and director of research for the Center on Philanthropy at Indiana University, which co-authored the Bank of America study. "They are more focused, more engaged through volunteerism, and their commitment is strongest when they are personally involved with the non-profits to which they give."
Just as in Rockefeller's time, religious organizations get the largest gifts, followed by education and health services.
When asked why they stopped giving to certain causes, the wealthy cited overly aggressive solicitations as the number one reason.
How much do you think is "enough" for the wealthy to give?
-By CNBC's Robert Frank
Follow Robert Frank on Twitter: @robtfrank