Holden's analysis seems sensible. To it I would add only two points:
1) Ratchet up your giving each time you get a higher income - at the time the new money is coming in, and before you start thinking about how to spend it. After you get a raise, especially an unexpected raise, is the best time to donate - rather than waiting to think about it at the end of the year.
2) Holden's analysis implies that if you have no income, e.g. as a student, you should wait to donate until later. This strikes me as basically correct but it is really really surprisingly important to give something on a regular basis - and put cognitive effort into efficient altruism / optimal philanthropy / rational charity, to get into that habit as well, even for small amounts.
At the end of every year, for example, you might donate $100 - or even $10, if $100 is too much - after looking over the latest list of efficient charities and doing some thought about where the $100 ($10) will do the most good, for purchasing utilons rather than fuzzies, just as if you were about to give $10,000. If the end of the year is far enough away and you don't have a trustworthy reminder system already set up, you might do that part now, then again a year later or at the end of the year, etc.
If you don't do this part, I would evaluate a surprisingly low chance that you would remember to start giving, and giving efficiently, later in life when you have income.
You might be able to get the habit-forming effect without "wasting" $100 or $10 by deciding how much you would like to donate in terms of your income and debt, then creating a worksheet for yourself which you dutifully fill out every month, even when you know it will come out to $0.
Topic: I will be in debt for several years, but will eventually have a disposable income. Should I donate now or later?
Here's my situation: I am a student, with student loans and no income. I can take out more loans than I need. Grad PLUS loans have a fixed interest rate of 7.9% - higher than, say, a mortgage rate, or expected stock returns. Some day, I will have those loans paid off, and will have money that I intend to give to charity.
My objectives: to live on less than my means, and give a significant fraction of my income to charity.
Question: When, if ever, should I give to charity before paying off those loans?
My initial reaction is to keep a record of how much I feel like I should be giving now, then give it later, adjusted for interest (at some rate equal to or less than 7.9%) - this would result in a bigger donation, but the same impact on my finances.
The only times I think I should give now, and not later:
1)If I don't believe I will make good on my commitment later on. (I'll presumably have a family and bills, etc, and while I am perfectly happy to live on little myself, I know I will want my kids to have nice things. This is somewhat illogical, but I'm imperfect.)
2)If the most worthwhile charity I find see has a higher interest rate than 7.9%.
3)If I find an opportunity to use my money charitably in which I can do more good than others, or where no one else can or will donate. (Mainly, random acts of kindness to strangers or friends - or, someone is matching my donation)
I doubt I will ever see 2) happen (or if it does, I should raise awareness)
3) doesn't happen very often, but when it does I think it is an acceptable use of funds
1) is the main scenario that concerns me. I've heard that "giving charitably is a habit" (that's why my parents had me tithe as a kid). I think that's true, though I haven't read any research on that. Either way, though, as I have no meaningful income (and my loan "allowance" is way more than I care to borrow), how much should I donate to help form the habit?
What do you think? Also, are there any other reasons to donate sooner and not later?
Edit: Givewell has an article on giving now vs later. Not all of it is relevant to my situation, but one point:
>"Economic growth, increased giving, and smarter giving may mean that giving opportunities are worse in the far future."