Meaning of "She sang when I ate . . ." in terms of time

Question

Suppose I ate from 7:00am to 7:30am.

If I then say:

1. She sang when I ate.

What time did she start singing?

If I said:

2. She sang when I ate for 30 minutes.

What time did she start singing?

If I said:

3. She sang when I ate until 7:30am.

What time did she start singing?

If I said:

4. She sang when I had eaten for 20 minutes.

What time did she start singing?

Answer 1

None of your options describes when she started singing. Because the time element comes after the eating, that's what it describes. Let's try some other variations:

She sang the entire time I ate

Could have started before and ended after. All we know is that every bite was accompanied by music

For 30 minutes, she sang while I ate.

Since it's stipulated that your meal was 30 min long, then we can infer that she started when you did, and ended when you did. Without that stipulation, we know only that during some portion of your meal (potentially covering the entire duration) she sang.

She started singing when I sat down to eat.

When I had been eating for 20 minutes, she started singing

We know when it began, but not when it ended

Answer 2

In your example of singing for one's breakfast the singer's singing may have happened at different times, the reference time frame is your breakfast

1. She sang when I ate.

She was singing sometime during the time between 7:00 and 7:30, we do not know when she started or when she ended, only that the two actions coincided with each other during the timeframe

2. She sang when I ate for 30 mins.
   She sang after I ate for 30 mins.

This is ambiguous, it is possibly #1 in a different guise, so it could have the same meaning as #1 or it could be interpreted to mean she started singing after you finished eating in which case the alternative sentence would be much clearer

3. She sang when I ate until 7:30am.

This is #1 in a different guise, we do not know when she started singing

4. She sang when I had eaten for 20 minutes.

The past perfect means that you finished eating for 20 mins (and may or may not have continued for the remaining 10 mins) and places it further in the past than the simple past
The singer started singing at 7:20

Answer 3

I could interpret some of that a little bit differently. It's less specific to particular duration of singing, but more about the frequency.

1. She sang when I ate.

Each time I ate, she sang.

2. She sang when I ate for 30 minutes.

She only sang when my eating took 30 minutes.

3. She sang when I ate until 7:30am.

She only sang when my eating lasted until 7:30am.

4. She sang when I had eaten for 20 minutes.

Once my eating got to 20 minutes she started singing. This is only one that specifies the time in your examples.

---

There's another way of phrasing these which lines up more with your original question, and that's by replacing "when" with "while" (or "whilst" for the British).

1. She sang while I ate.

She sang during the time I was eating: maybe the whole time, maybe less; it's unclear.

2. She sang while I ate for 30 minutes.

She sang during the time I was eating, but only for 30 minutes. It's unclear exactly when she started/stopped exactly.

3. She sang while I ate until 7:30am.

She sang during the time I was eating, and stopped singing at 7:30am. It's unclear what time she started.

4. She sang while I had eaten for 20 minutes.

This one actually sounds more convoluted, and is very unclear. I wouldn't use this.