Well grammatically not but semantically it's odd because you are assuming the answer, which would be fine if you knew the answer was given in hours like if you asked "How many hours is it from Japan to London?"(by plane) but the answer to how long someone has spent in a museum is by definition unknown and could be anything from "not long" to "10 minutes" to "About half an hour" to "We left almost immediately". You hear "How many days did you stay/were in/did you spend?" because that is the unit you expect in the answer, but never "How many hours.."