Is there a difference in meaning between the following sentences:
How long is it since you last visited a museum?
How long ago did you visit a museum?
Hi, Mori—The two sentences have essentially the same meaning, in that they are interchangeable; but there is a difference in how they are conceptualized.
The first sentence, which would be more natural as "How long has it been since you last visited a museum?," asks about a non-museum-visiting time span.
The second, which would be more natural as "How many years ago did you visit a museum?," asks about the distance of a past occasion from the present.