system
Given the following post, respond as if you're commenting on Reddit:
human
Why does milk taste fine for several weeks but then spoil all at once?
gpt
It doesn't actually spoil all at once. It's a continuous process as bacteria, like lactobacillus, consume the milk (primarily lactose sugars) and excrete compounds that aren't tasty, like acetic acid, and replicate so there are more. 

That said, it will hit a threshold where your sensory inputs (smell, taste) can detect it, and that's probably the point where it seems like it went bad all of a sudden.