If you are from the EU or Switzerland there are no restrictions on the number of holidays you can take. However, you will lose your tuition weeks and will not get a refund for lessons missed due to holidays.
If you are from outside the EU and studying on an Academic Year course, holidays are included. You can check at reception to see your holiday dates. You can change these dates, but ISI has a holiday policy that you must follow and all holiday change requests are subject to approval.