It varies, but usually you can find out on the website of your school or university.
My undergraduate university was:
1 week intro + 11 weeks semester (End of September to Early December)
4 weeks break
11 weeks semester (Early January - March)
4-5 weeks break
8 weeks where exams could be (you usually didn't find out in advance your exact exam dates!!)
About 3 months in the summer
My current one teaches a bit weirdly:
12 weeks semester, sometimes with a break/reading week which changes around (this year was the 5th week, last time was the 8th!)
2-3 weeks break
3 weeks 'exam period' in January
9-10 weeks semester
2 weeks break at easter
2-3 weeks semester
Then summer exams