I believe that charity is morally incumbent upon each one of us. Not just generosity, for that is largely dependent on one's nature. The frequency of giving is predicated on things like ability, opportunity and environment. But charity is, for me, something that is required for the purification of one's soul. Once you begin to give because someone else needs it, it ennobles your spirit. When you give of something you cannot spare, it demonstrates how immaterial material things are. It makes you more than the sum total of your possessions. That is a very empowering feeling.
That also means that charity is independent of a person's resources too. How much you have has no correlation to how much you can change someone else's situation, because you will be fulfilling someone's need at that moment. Maybe it is $10, maybe it is a $1000, or maybe it is a snow shovel to borrow. Or giving a push to a stalled car.
I think it is charity when you can smile and wish someone a good day when all you feel like is kicking something or someone(!) real hard. You have to draw on draining emotional reserves to do the right thing. I think that is what the Prophet Muhammad meant when he said 'Smiling is charity.' He was not talking of the mindless grinning to look good, or smiling to look pleasant and make an impression. He was talking of smiling to make the other person feel better.
Oscar Wilde said that no good deed is unselfish. Everything we do is to make ourselves feel better. Are we all so jaded that that is what the definition of good is reduced to? Real charity, I have always believed, is something constant - irrespective of your means, situation, or your life plans. If you are giving $100 to a soup kitchen when you earn 5000K a month, please do not donate a million to UNICEF when you get that 15 million dollar lottery. Make the equal percentage donation to what you know in the heart you always needed to do.
I guess I may be envious of people who have too much money and time to know what to do with themselves - the royalty by accident of birth, the rich by marriage, even the stock market gamblers. So when I accept their generosity at face value as 'charity' - that they are doing this for others and not for themselves, it is I who is being my charitable best!