5

One was not permitted to drink liquids that were left uncovered.

Some (like the Vilna Gaon: גלוי אסור אף שאין נחשים מצוים בינינו. והוא ז״ל היה נזהר מאד מאד בדבר) was stringent even in countries where there are no snakes (like in Europe).

How long is considered "left out"?

8

See e.g. Maimonides Hilkhoth Rotzeach U'Shemirath Hanefesh 11:8

כל המשקין האסורין משום גילוי שנתגלו בין ביום בין בלילה אסורין ואפילו היה בצדן אדם ישן אין אימת ישן על הזוחלין. וכמה ישהו ויאסרו כדי שיצא הרחש מתחת אזן כלי וישתה ויחזור למקומו

When a liquid is forbidden if left uncovered, it is forbidden whether it was left uncovered during the day or during the night. This applies even when a person was sleeping beside the liquid. For crawling animals are not afraid of sleeping men.

For how long must a liquid be left uncovered to be forbidden? For as long as it takes for a crawling animal to emerge from under the container, drink, and return to its place.

Based on Terumoth 8:4

יַיִן שֶׁל תְּרוּמָה שֶׁנִּתְגַּלָּה, יִשָּׁפֵךְ, וְאֵין צָרִיךְ לוֹמַר שֶׁל חֻלִּין. שְׁלֹשָׁה מַשְׁקִין אֲסוּרִים מִשּׁוּם גִּלּוּי, הַמַּיִם וְהַיַּיִן וְהֶחָלָב. וּשְׁאָר כָּל הַמַּשְׁקִין מֻתָּרִים. כַּמָּה יִשְׁהוּ וְיִהְיוּ אֲסוּרִין, כְּדֵי שֶׁיֵּצֵא הָרַחַשׁ מִמָּקוֹם קָרוֹב וְיִשְׁתֶּה:

If wine that is Terumah is left uncovered, it must be poured out. It is not necessary to say this if it is Chulin [non-sacred produce]. Three kinds of liquids become forbidden if left uncovered: water, wine, and milk. All other liquids [even if left uncovered] are permitted. How long must they be left [uncovered] for them to become forbidden? As long as [it would take] for a snake to come out from a nearby place and drink [from it].

4

The Aruch Hashulchan at the end of Yorah Deah 116:3 writes (emphasis mine):

וכמה זמן ישהו שיהא מגולה ותיאסר? כדי שיצא הנחש מתחת אוזן הכלי ויחזור לחורו, והוא זמן מועט

And how much time can the drink be left uncovered until it becomes forbidden? Enough time for a snake to come out from under the handle of the vessel and return to its' hole, and that is a short time.

While still not giving an exact timeframe, the Aruch Hashulchan is at least clarifying that it's a relatively short amount of time.

In the most recent issue of Kashrus Kurrents published by the Star-K (Spring 2019) they have an article about various food-related dangers and the last issue they mention is uncovered drinks. According to the article (which quotes a work named שמירת הגוף ובנפש as the source):

The timeframe in which to forbid uncovered liquids is three seconds.

  • @WAF You're right, I edited in 'handle' to the translation. Couldn't think of the right word when I wrote this up last night. The main reason I wrote this (since at the time Loewian had already provided a clear answer) is the last 3 words which stresses that it's a 'short time'. While still a vague amount of time, that at least clarifies somewhat that it's not a huge amount of time. – Salmononius2 Oct 23 '17 at 14:39
3

The gemara in Chulin 10a says the amount it takes for a certain creature (snake acc to some like mentioned in other answers) to come out from under the handle and drink and return back to its hole without being noticed.

Tosfos brings from the Yerushalmi that there is a creature which is as thin as a hair and it's called a shififon,and it has the ability (reshus) to penetrate ground but not vessels. So the time it takes for this creature to crawl from the handle into the drink

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