A simple Google search on how much to invest will overload you with advice--for 401(k)s, IRAs, stocks, bonds, commodities, beanie babies, whatever. But a similar search on giving offers up meager results at best (including some links about rat poison).
There is a lot of talk about smart giving, but not a lot of advice on how much money individuals should fork over after vetting their organization. The do-gooder answer is simple: more, more, more, but that doesn't hold much weight in a country with high-unemployment, tight budgets and normal people wanting to make a difference.
For those who live a life of excess, like the demon bankers of Wall Street, the mantra of more can be a good financial guide (or shame tactic). But for those of us debating between donating $100 and $200, or even $25 or $50, it would be good to know the optimal levels of giving.
Luckily, Peter Singer, a philosopher whose most famous work is The Life You Can Save, created a giving calculator for this specific purpose. Just type in your income and boom! there's your charitable donation for the year. I entered in mine, $12,000, and it spit out $120.
Now, I like to think of my self as generous, but, based on my stipended-volunteer budget, my expected donation is 120% of my monthly personal spending, which covers everything but room and board. I looked into the rational behind these figures and found this explanation. The levels for giving Singer devised seem a little arbitrary.
And I think that's what the answer to the question "how much should I give?" will always be: arbitrary. Asking that question assumes many things: First, that your donation (investment) will be used effectively and efficiently. Second, your donation (investment) will be used with similar amounts of effectiveness and efficiency across all causes and across all organizations. Third, you will be able to measure a return on that donation (investment) that will justify the amount of money you put in.
As it stands now, only the first of those assumptions holds, and only in certain cases. Until all those assumptions are reality, any guide on how much to give won't hold a lot of weight. One day, social impact measurements may be able to make the other two assumptions true, but I doubt we will ever (or even should) posses the tools to accurately measure social change in quantifiable amounts. (More on that later, but until then, read this.)
So, my answer to the "how much?" question is a modification, but as equally a cop-out, on the do-gooder answer. You should give what you think you can. Pushing your limits is never discouraged, but ultimately, because the return on social investments is unquantifiable social change, you are the only one who can decide what you can afford.
Unless you are one of those people living a life of excess. Then, you don't really have an excuse.