How did "When" become the customary answer to "say when"?

When a waiter at a restaurant comes by with pepper or Parmesan cheese, he says, "say when" and starts putting it on your food. Many people will say "OK" or "that's enough," but it seems that the customary answer is "when". How did this become the customary answer? Did everyone decide to make the exact same joke, until it became commonplace. I thought it was funny when I was a kid, but even now as an adult, I still say it.

For evidence that this is indeed commonplace, see http://www.gocomics.com/broomhilda/2012/06/20. The answer "when" is used and is external to the joke of the strip. But even so, I'm curious as to just how commonplace this really is. I see it a lot in the US. Is it used elsewhere in the world?

(by the way, I'm new to asking questions on this site, and I have no idea what tags to put on this question)

I can't answer how, but I can say when.

According to the OED:

say when, colloq. formula used by a person pouring out drink for another, to ask him to say when he shall stop; also ellipt., as a reply to this formula.

The question is at least from 1889 and the answer from at least 1911:

• 1889 John S. Farmer Slang and its analogues past and present: ‘Say when,’ said Bonko, taking up a flagon of whiskey and commencing to pour out the spirit into my glass.
• 1911 Maclean's Mag.: ‘Say when?’ I held the glass with a shaking hand: ‘When.’
• 1931 A. Powell Afternoon Men: ‘Say when, sir,’ said the waiter. ‘When,’ said Pringle.
• 1948 E. Waugh Loved One: ‘When,’ he added aside to the young man, who helped him to whisky. ‘Right up with soda, please.’

I found an earlier example of both question and answer in Rudyard Kipling's A Conference of the Powers (1890):

Following the first great law of the Army, which says ‘all property is common except money, and you’ve only got to ask the next man for that,’ The Infant offered tobacco and drink. It was the least he could do; but not the most lavish praise in the world held half as much appreciation and reverence as The Infant’s simple ‘Say when, sir,’ above the long glass.

Cleever said ‘when,’ and more thereto, for he was a golden talker, and he sat in the midst of hero-worship devoid of all taint of self-interest.