Depends on when you'll finish all the recommended coursework for it and when you'll be able to prepare adequately. Taking it January would give you time to retake without a gap year if your scores turn out low. However, taking it in late spring or in summer would let you prepare more with the risks of having to take a gap year if you don't score well and possibly being unable to apply early. I've known people who've also taken it summer after Sophomore year since they had finished the prereqs and prepared adequately. Moral of the story, before senior year if you don't wanna do a gap year.