You have been admitted to the United States as an F-1 student or J-1 exchange visitor for "duration of status", noted as "D/S" on both your I-94 card and I-20ID (for F-1 students) or IAP-66 (for J-1 exchange visitors), Certificate of Eligibility. "Duration of Status" means you may remain in the U.S. for the period in which you are pursuing a full course of study. At Purchase College you must be enrolled for at least 12 credits each semester to be in full-time status if you are an undergraduate and 9 credits if you are a graduate student.
If you are a full time student and follow all the rules for F-1 students or J-1 exchange visitors, it does not matter if your visa expires while you are in the United States. When you travel out of the United States and your visa has expired, you must travel to your home country to obtain a new visa at the U.S. embassy there, so you can re-enter the U.S. as a F-1 student or J-1 exchange visitor. (See the section on "Rules for Travel").