Hello Sathi, F1 visa never expires as long as you are in this country in your status. Thatz the reason you can see D/S- durations of status in your I-94. Which means, the visa is valid as long as you fulfill your status, here go to school full time. The only time you will need a new visa is if you have to reenter this country. So the best solution for you is if you are not planning to go back home any time soon. Dont even worry about going to Canada or Mexico.