One of my friends went on a trip to Hong Kong, and I wanted to know for how many days, the complete question could be…
How many days did you go to Hong Kong?
How many days did you stay in Hong Kong?
How many days did you spend in Hong Kong?
How many days did you visit Hong Kong?
I would use “stay” or “spend.” “How many days did you go to Hong Kong?” sounds like it’s more about length of the journey there than the time in Hong Kong. “How many days did you visit Hong Kong?” has the same problem. It would sound better if you said “How many days did you visit in Hong Kong?”, I think.
How long were you in Hong Kong?
How many days did you stay in Hong Kong?
How much time did you spend in Hong Kong?
How long was your visit to Hong Kong?
p.s.
I would probably only ever use the phrase “did you go to Hong Kong” in a few instances.
To ask whether someone had gone to Hong Kong:
Did you go to Hong Kong?
Same with “did you visit Hong Kong” if I already knew they had gone to China, but I wanted to know whether they had specifically visited Hong Kong:
Did you visit Hong Kong?
Or to ask when someone went to or visited Hong Kong:
When did you go to Hong Kong?
When did you visit Hong Kong?
Of course there’s also the question ‘why,’ but I would probably only ever find myself asking such a question if I had forgotten the purpose of their trip:
Why did you go to Hong Kong again?
Why did you visit Hong Kong again?
(using ‘again’ to mean ‘could you tell me again the reason why you went, because I forgot’ rather than ‘why did you go an additional time?’)
But wait, lilyyang! I really didn’t think this one through at all. See dcj201’s comment above.
I’m not sure what the right term for this is, but I think dcj201 is exactly right. The questions you wrote are all acceptable. I think it’s just a matter of personal preference and not necessarily grammar. There are probably more ways to ask the same question than any one of us could think of.
I guess if I wanted to specifically ask ‘how many days,’ my personal preference would be, ‘How many days were you in Hong Kong for?’ rather than, How many days did you visit/go to/stay in Hong Kong for? or How many days did you spend in Hong Kong? (the only one of the four question where ‘for’ doesn’t fit.)
I would just simply ask, “How long were you in Hong Kong?”