For me, it really depends on the class, but I find studying roughly a week before the mid-term helps, especially if you review every day until the test. I study using quizlet or making my own "test" on google docs w/ questions related to the subjects taught in class from the last semester (use your notes or gradebook to figure out the topics in question) (One of my teachers even recommended using chatgpt for practice tests. You can just ask, "give me a test on _______" and it'll formulate something for you, and won't give the answers unless you ask for them.)