Pritati, You got 15 days to pack your bags and get outta this country after you end yor semister. After 15 days, if you are still in this country you will be consider illegal. So, if you want to come back here again, I would suggest to leave within that 15 days. if you leave after 15 days but before 180 days (6 months), I think you will have 3 years bar from applying for US visa again. If you leave after 180 days, you will be under 10 years bar.