To Gain and to Give: Hilchot Tzedakah, Day 23
How often should one separate ma’aser? One should separate ma’aser at least once every six months, or at the very least once a year. Often, one’s earnings/losses can fluctuate with the passage of time. Calculating one’s profits and the amount of ma’aser to give is determined by the date which one sets to take ma’aser.
אהבת חסד פי"ח ס"ב, צדקה ומשפט פ"ה בהערה