How many hours is 3pm to 10pm?

Answer 1

Your question has no answer because there is no time from 10pm to 3pm. If you mean 3 am it would be 7 hours. Unless you were asking about 3pm the next day, but your question gives us no clue that is what you are asking.

Answer 2

My mistake; I previously read the question incorrectly.

22:00 (10 PM) to 15:00 (3 PM) the following day is 17 hours.