In an AC circuit, where does hot end and neutral begin?
While I don't necessarily like using plumbing as an analogy for electricity, it can be helpful in this situatuon. 

Think of a kitchen sink as an electrical device, or load. The supply lines are "hot", while the drain is "netural". The supply lines have pressure, which forces the water out of the tap. Once at the sink, the pressure is released.  The water then flows down the drain with no (very low) pressure. 

It's a similar situation in the electrical world. The "hot" conductors have a "pressure" (electrical potential). Once the electrical energy is used by the device, it flows down the "drain" ("neutral") conductor at very low pressure (electrical potential).

To answer your question directly. The "neutral"; or grounded conductor, "begins" after the load. Where the electrical potential is near zero (relative to ground).