A semester is typically about four months long, usually ranging from 14 to 17 weeks of instruction depending on the institution. Details:

• Most US colleges split the academic year into two main semesters: fall and spring. Each semester commonly lasts around 15 to 17 weeks, with final exams in the last week or two.

• Some schools use shorter or longer variations:
  • Condensed or accelerated semesters can be 8 to 12 weeks long.

* Quarter systems divide the year into four terms of about 10 to 13 weeks each, with summers optional.

• Summer terms, when offered, are typically shorter (often 8 to 12 weeks) and optional for many students.

If you have a specific institution in mind, I can look up its exact semester length and calendar.