The atmospheric lifetime of CO2 is usually given as being 30-90 years, but CO2 is taken up by the environment more slowly over time and it's estimated that 20% of our CO2 emissions will remain in the atmosphere for 1000 years or more.
Here's a paper specifically on CO2 and some excerpts from wikipedia.
https://www.google.com/url?sa=t&source=web&rct=j&url=https://www.coursera.org/lecture/global-warming/the-long-co2-tail-TKa5w&ved=2ahUKEwiXgKPlvcTuAhVORqwKHfyWB1cQFjACegQIBxAB&usg=AOvVaw1kOXQsj6xc8U9YdGLPY6Wqhttps://en.wikipedia.org/wiki/Greenhouse_gasAtmospheric lifetime
Aside from water vapor, which has a residence time of about nine days,[32] major greenhouse gases are well mixed and take many years to leave the atmosphere.[33] Although it is not easy to know with precision how long it takes greenhouse gases to leave the atmosphere, there are estimates for the principal greenhouse gases. Jacob (1999)[34] defines the lifetime {\displaystyle \tau }\tau of an atmospheric species X in a one-box model as the average time that a molecule of X remains in the box. Mathematically {\displaystyle \tau }\tau can be defined as the ratio of the mass {\displaystyle m}m (in kg) of X in the box to its removal rate, which is the sum of the flow of X out of the box ({\displaystyle F_{out}}F_{{out}}), chemical loss of X ({\displaystyle L}L), and deposition of X ({\displaystyle D}D) (all in kg/s): {\displaystyle \tau ={\frac {m}{F_{out}+L+D}}}\tau ={\frac {m}{F_{{out}}+L+D}}.[34] If input of this gas into the box ceased, then after time {\displaystyle \tau }\tau , its concentration would decrease by about 63%.
The atmospheric lifetime of a species therefore measures the time required to restore equilibrium following a sudden increase or decrease in its concentration in the atmosphere.
The average time taken to achieve this is the mean lifetime.
