BBC - Climate Change: The Blog of Bloom claims a really long life for atmospheric CO2

...basically because there's much less methane and it has a shorter life in the atmosphere - roughly 8.5 years to CO2's roughly 100 years.

A different estimate: CO2 ACQUITTAL (Rocket Scientist's Journal)

[The residence time of CO2 is easily calculated from IPCC (Consensus) data. It is about 1.5 years to 2.0 years, depending on whether you include the leaf water uptake reported by the IPCC (Consensus).