It depends. Some pts are in the ICU because of the severity of their disease. They might need invasive ways to monitor their vital signs, which produce a continuous monitoring (i.e. Invasive BP monitoring). Some other pts are there because they may have a complication in the next few hours and they require a more often monitoring than in the regular floors, usually once per hour (i e some post-op pts).
Continuously. In most cases, the vital signs are monitored continuously. However, the data may only be recorded with a certain frequency, often every hour. Obviously, the bedside nurse is keeping very careful scrutiny over his/her patients' vitals. The nurse may choose to record vital signs to evaluate a trend (e.g. Increasing heart rate).