Terumoth 8:4 implies that the drink would only have to be unobserved for a short time to be forbidden:
כַּמָּה יִשְׁהוּ וְיִהְיוּ אֲסוּרִין? כְּדֵי שֶׁיֵּצֵא הָרַחַשׁ מִמָּקוֹם קָרוֹב וְיִשְׁתֶּה:
How long must they be left [uncovered] for them to become forbidden? As long as [it would take] for a [creature] to come out from a nearby place and drink [from it].
According to the Yerushalmi (8:3), cited by the Rash MiSens, the amount of time allowed uncovered is very short, the "nearby place" including the handle of the vessel containing the fluid.
The Bavli (Chullin 10a) seems to only extend this allowable period slightly:
וכמה "מקום קרוב"? א"ר יצחק בריה דרב יהודה: כדי שיצא מתחת אוזן כלי וישתה
And how far away is considered a "nearby place"? Rav Yitzḥak son of Rav Yehuda said: [Even a period equivalent to] the time [necessary so] that it could emerge from beneath the handle of [the] vessel and drink.
ישתה" - הא קא חזי ליה? אלא ישתה ויחזור לחורו":
[The Gemara asks: If it is only the time necessary for the creature to emerge and] drink, doesn’t one see [the creature drink, in which case there is no uncertainty]? Rather, [it is a period equivalent to the time necessary for a creature to emerge from a proximate place,] drink, and return to its hole. [If one left exposed liquid unattended for that interval, it is possible that the creature drank the liquid unseen by the owner of the liquid.]