The phrase "long lifetime" refers to the amount of time that someone or something can live or exist. It means that they have a very long span of time before they come to an end or reach the end of their existence. Full definition
Indeed, «goals» and «caps» on carbon emissions are practically worthless, if coal emissions continue, because of the exceedingly long lifetime of carbon dioxide in the air. (climatesciencewatch.org)
Unfortunately, the carbon dioxide being released today has a much longer lifetime in the atmosphere than you think. (skepticalscience.com)
Once in a lifetime — once in a very long lifetime — the political stars align. (theguardian.com)