Merriam-Webster's Collegiate® Dictionary, 11th Edition
LIFETIME
I. noun
Date: 13th century
1. the duration of the existence of a living being (as a person or an animal) or a thing (as a star or a subatomic particle), life 12, an amount accumulated or experienced in a lifetime ,
II. adjective
Date: 1834
lifelong , of long duration or continuance , measured or achieved over the span of a career