How often are vitals monitored in icu?

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).