Small difference with the word need versus should. Need is a requirement, something you must do. Should is not a requirement but more of an expectation. You didn’t need to spend so much time would mean you spent more time than was necessary. Like if you worked on an assignment for three hours when everyone else only took an hour. You shouldn’t have spent so much time on it means it would have been better if you used your time differently. Like you shouldn’t have spent so much time with your friends because your mom needed help at the house.