When to give money to Charity?

I am hoping to one day have a blog that earns money purely for charity.

For now I don't quite earn enough to give vast amounts to charity but I really hope that I can get to a certain earning level and then start giving a percentage to various charities each month.

What level that is.... I don't quite know yet. I guess I need to work out an amount over all my bills and then anything above and beyond that should leave my hands.

When do you decide to give to charity?