# 'How long' - (duration of an activity) for past simple continuous

'How long' for sentences in the past simple continuous to talk about the duration of an activity in the past?

• How long did it rain last night?
- It rained for about 3 hours.
• How long did he study yesterday?
- He studied for six hours.

I know that we can use the duration with simple past tense to talk about the completion of an activity in a sequence. However, in the past simple continuous,

• How long was he studying this time yesterday?
- He was studying for six hours.

can we use past simple and past simple continuous in some sort of interchangeably?

As we are not talking about an activity that went on up to that time, in this case, we use past perfect, but we are talking about the sequence of actions. For example,
Yesterday, I went to a restaurant. And I waited for my friends for an hour/ I was waiting for my friends for an hour, but none of them came, so went back home.

I think it's just about nuances. Using a simple emphasis on the completion and continuous emphasis on the ongoing activity.

• Yes. Note that "for" can be added, as in "For how long did it rain last night?, or How long did it rain for last night? Commented Feb 25 at 11:24
• Note idiomatic "How long is a piece of string?" used as a dismissive response to an "imprecise" badly-posed question (until you specify which piece of string, the length is indeterminate). Commented Feb 25 at 11:55
• The third sentence doesn't work because this time yesterday refers to a fixed point in time. Commented Feb 25 at 14:09
• I'm not sure what meaning is intended by the third example. I suspect it intends something like "How long had he been studying for by this time yesterday," but please edit to make it clear. There's nothing wrong with the other two, but please edit to tell more about why you had doubts about them. Commented Feb 25 at 22:50
• @AndyBonner edited Commented Feb 26 at 9:42

Yes, you often use how long to ask questions about duration of time. It can be used to ask questions about the past, present and future. You can clarify when you're asking about, or leave it up to the context of the discussion. Your examples are correct, except for the last one:

How long did it rain?
How long did it rain last night?
How long did he study?
How long did he study yesterday?
How long was he studying this time yesterday?

Other examples:

How long is the movie?
How long have you been sitting here?
How long will you be in California?
How long will you have been married on December 31?

Note that it can also be used to ask about an object's length, for example:

How long is a yardstick?