This is a highly subjective matter.
From the time the cap is removed the first time,
airborne bacteria begin to reproduce in the milk.
But here is where chance comes in- Did the cap get dropped?
Did your hand touch the underside of the cap at any time?
Did your breath land on the inside of the container when you opened it?
These seemingly meaningless questions make the difference between milk that goes bad quickly and not so quickly.
Every time the milk is opened,
it is innoculated with new bacteria.
How fast they reproduce and how much toxin they produce is randomly up to thier species,
of which there are millions,
and this is not mentioning the fact that it could have sit unrefrigerated for an unknown period of time while being brought to your local store`s shelf.
The answer is You don`t and can`t know.
Pour the milk out and get a fresh one after the glass stops sweating