That is dependent on your university norms or teaching faculty. You may receive the academic credit for participating in internship program for which you need to contact your teacher and mentors who can help you find the process.
There is no such expiry date for the internship. For a student, it is the most valuable certificate that tells about his/her calibre and duration of internship.
Yes, Spring breaks and winters are best time to complete your internships. It gives you ample of time to concentrate on the project and do research on the same.