For how long can a person be safely under conscious IV sedation during a lengthy dental procedure?

This depends on many factors, including the age of the person and their health. As long as heart rate, oxygen saturation, and other vital signs are closely monitored, people can safely be sedated for extended periods of time, in some cases weeks or months (or in very rare and extreme cases, years). Such extended sedation, however, can have its own side-effects upon waking, most notably muscular atrophy from lack of use.

However long a dental procedure may take, the patient would probably be safe if his or her vitals are monitored appropriately and care is taken when appropriating and administrating anesthesia.