The SA websites have info. One semester, usually 2/c year. Your major may impact whether it’s a viable option, depending on required courses offered. You also have to be a cadet or midshipman in good academic, performance, conduct, etc., standing. If you’re struggling academically (someone in each class gets the C, D and F grades), on remedial for physical fitness or have a demerit surplus, you’re not someone your SA wants representing at another SA.