Include a snorkel with your gear when diving the Rainbow River. Egress/access is limited, some folks burn more air than they think they would due to the cold and current: don't fight the current, but you will experience some counter current conditions as you swim from hole to hole and across the river. You may get low on air before your exit, so a snorkel can help you enjoy the dive.
Headsprings to KP is about 75 minutes if memory serves. KP to Swampy's Bar is two and a half to three hours, IIRC.