There is a tradition of Maaser Kesafim, to give 1/10th of ones salary to charity. Is there a particular set time when the obligation to set aside this money for charity begins? For instance, is one obligated to set aside Maaser right away upon receiving a pay check, or does the tradition have no exact set time of obligation and as long as once a year or once in a while one checks to see how much Maaser is owed it is sufficient for the obligation?
As I explain in More Maaser and Tax Math and as can be seen in Maaser Kesafim: Giving a Tenth to Charity this is the equivalent of having made a neder to give a minimum percentage to tzedakah. Thus, one is obligated as soon as the income is received. However, one can maintain a ledger of income as it comes in and tzedakah as it is distributed so that one can be sure that one has not missed giving sufficient tzedakah. It is obvious that one cannot sent out tzedakah at the instant the income is deposited in the bank, so it is only the ledger that needs to be kept up to date so that one knows how much is "owed" to tzedakah at any time. One should answer tzedakah mailings as soon as possible or give checks to meshulachim whenever they come to the door.
The title of your question speaks of obligation while the text of your question speaks of tradition. There is indeed a significant discussion amongst poskim about the exact status of maaser. R Avrohom Chaim Feuer explains in "The tsedakka treasury" that some believe it is a Torah obligation (e.g., Yoreh Deah 232, Maharil as cited in Responsa Chatam Sofer), some believe it is a rabbinical mitzva while the majority of poskim (e.g., Bach, Yoreh Deah 331, Chatam Sofer) sees it as time-honored minhag which should be adopted by all.
The rulings I found (in R Feuer's book as well as in R Shimon Taub's The laws of tsedakah and maaser) allow yearly accounting of once's maaser, i.e., you do not need to give maaser as income arrives as long as you "clear the account" regularly.
The Chofetz Chaim (in Ahavas Chesed 18:2) offers the following guidance on setting up a maaser system (with personal additions to make it practical to the 21st century)
- Set up a separate ledger (or a spreadsheet) to track earnings, any business expenses, taxes and donations
- Designate a specific date (e.g., Rosh Hashana) to mark the end of an accounting period where you "close the books" - should be at least once a year - can be more frequent
- At the time of the closing period, calculate profits, deduct losses and acceptable expenses and the amount of maaser
- Deduct all donations already made and distribute immediately whatever you owe (i.e., the difference between maaser and previous donations)
- If you have given more than your planned maaser, some authorities allow to carry over to the next accounting period. Even those that are strict allow it if you stipulate (orally) that you may deduct from your maaser obligations at all times any donations you make at any time you desire