Amount of Tzedaka



How much Tsedaka should I give(Tsedaka is not given in person. It is given to an online organization via credit card). I read somewhere that one should give one fifth of something) - It is not clear to me at all. - Is it one fifth of my weekly salary/monthly salary? And what if my salary is different every time?
And also - how often should I give tsedaka??


To the Questioner,

The custom is to give one tenth of one's earnings. 

The first time one practices this custom, he estimates one tenth of his present liquid monetary ownings and gives it to Tzedaka; 
following that, each time he receives income, he estimates 10% of the income and gives it to Tzedaka.

With Blessings, 
Rav Nachum