What is the difference between "how long ago" and "when"?

Question

I am really interested to know the difference between these two phrases

How long ago did you ... ?

and

When did you... ?

I wonder if they both are similar or different or are used rarely or common. Would you mind explaining to me this please?

Answer 1

If you ask how long ago, you would get the answers with a number and a time unit, like one year, 3 weeks, etc.

If you ask when, you would get a specific time / date.

Meanwhile you can use when to ask events happen in future.

Answer 2

When did that happen?
How long ago did that happen?

These are both questions that ask about some event in the past. They are looking for an answer that provides some time reference between the present and some previous point in the past (like, "three months ago," or, "last winter", or, "on the fifth of May," or "in 1492.") It could be long ago, or recent history:

When did Magellan sail around the globe? (Answer: In the early 1500s.)
When did you start having chest pains? (Answer: Twenty minutes ago.)

But there is rarely just one way to say something in English. So we could also ask (and answer) in this way:

How long ago did Magellan sail around the globe? (Answer: About 500 years ago.)
How long ago did you start having chest pains? (Answer: They started around 6 o'clock.)

Note: I've given different answers just to show there's no single way to do answer these questions. The answers didn't changed just because the questions changed. For example, all of these examples show normal, grammatical, idiomatic speech:

When did Magellan sail around the globe? (Answer: About 500 years ago.)
When did you start having chest pains? (Answer: They started around 6 o'clock.)
How long ago did Magellan sail around the globe? (Answer: In the early 1500s.)
How long ago did you start having chest pains? (Answer: Twenty minutes ago.)

So, in some ways, they are interchangeable, but in other ways they are not.

For example, you can use when to ask about the future as well as the past:

When did you go to Italy? (Answer: Three months ago.)
When will you go to France? (Answer: Next summer.)

whereas how long ago must change when you talk about the future:

How long ago did you go to Italy? (Answer: Three months ago.)
How long from now until go to France? (Answer: Next summer.)

An interview might go something like this:

When did you write your first song?
When I was 16.
How long ago was that?
Well, I'm 38 now.
So, more than twenty years ago, then.
Yes, it's been twenty years. Wow, time flies.

If that interview started differently, though, the interviewer's second question might change a little bit:

How long ago did you write your first song?
When I was 16.
Oh, and how old are you now?
I'm in my late thirties.
So, about twenty years ago, then.
Yes, it's been a little over twenty years. Wow, time flies.

As for which is more common, the version with when is more common. It's simply a shorter, more common word that's easy to use, and quite flexible.