For what percentage of Earth’s history has life existed on Earth?
Taking Earth's age as nearly 4.6 Billion Years (BY), instead of 13.7 BY from Big Bang, unicellular life appeared nearly 3.8 BY ago. This percentage is 3.8/4.6 X 100 = 82.6%, nearly.
Reference: www.bbc-co.uk_nature/history_oftheEarth. Before oxygen appeared, oldest growing and dividing microbe appeared on Earth nearly 3.8 BY ago. This is attributed to discovery of strong absorbing of ultraviolet rays that is correlated to the presence of hydrocarbons.