What are the exact halachos of mayim megulim? How long can the water be uncovered? How large can the opening be for it to become assur? Is it only overnight?
For those who are particular concerning the prohibition of uncovered water, water should not be left unconvered form the time it takes for a snake to travel from the handle of the vessel (or from the nearby vicinity), to drink of the water or liquid, and to return to its hiding place — a very short time. The prohibition applies to an opening the size of the little finger of a newly born baby. It applies not only overnight, but even in the day (Rambam, Rotze’ach U-Shemiras Hanefesh, chap. 11).
There are many more halachos of uncovered liquids, such as which liquids the prohibition applies to, and other circumstances in which the prohibition does not apply.
Many poskim (including Shulchan Aruch) rule that in places where snakes are uncommon there is no need for caution regarding this matter, though other authorities rule that one should nonetheless fulfill the instructions of Chazal.