"How long does carbon dioxide remain in the atmosphere?
CO2 lifetime in the atmosphere can range from 50-200 years. Once it enters the atmosphere, there is very little that can be done to reduce carbon dioxide in the atmosphere."
http://www.ecobridge.org/content/g_faq.htm

"The notion is pervasive in the popular and scientific literature that the lifetime of anthropogenic CO2 released to the atmosphere is some fuzzy number measured most conveniently in decades or centuries. The reality is that the CO2 from a gallon out of every tank of gas will continue to affect climate for tens and even hundreds of thousands of years into the future.

The U.S. Environmental Protection Agency Inventory of U.S. Greenhouse Gas Emissions and Sinks (2005) has the CO2 lifetime listed as 5-200 years...."
http://www.realclimate.org/index.php?p=134

And goes on to say:
"My model indicates that about 7% of carbon released today will still be in the atmosphere in 100,000 years [7]. I calculate a mean lifetime, from the sum of all the processes, of about 30,000 years. That's a deceptive number, because it is so strongly influenced by the immense longevity of that long tail. If one is forced to simplify reality into a single number for popular discussion, several hundred years is a sensible number to choose, because it tells three-quarters of the story, and the part of the story which applies to our own lifetimes."

"For one thing, where does Hansen get off calling PlayStation simulations "climate studies"?

For another thing - and this demands some focus - Hansen claims that a fifth of CO2 stays in the air for a thousand years. Okay, the IPCC puts each year's natural carbon emissions in the neighborhood of 200 billion metric tons. By Hansen's metric, this means that the oceans, dissolving rocks and plants will not see 40 billion of that till a thousand years have ended. Yet the IPCC also says that more than 98% of all carbon emissions - artificial and natural combined - are re-absorbed by oceans, dissolving rocks and plants each year."
http://tomnelson.blogspot.com/2008/01/what-is-average-lifetime-of-atmospheric.html

And goes on to conclude:
10 years, more or less. Every empirical study puts that as the average lifetime of atmospheric CO2.

And as I recall it was something like 10-12 years.
But if you followed around a particular CO2 molecule it might be absorbed in a minute or bounce around for a very long time.
If you standing in greenhouse during the day time most of CO2 you exhale it will absorbed in minutes- though a bug might eat part of the plant with that CO2 molecule as a sugar molecule and exhale it a few hours later- or the bug is eaten, or it dies and gets entombed for thousands of years, or it becomes part of baby bugs, etc.

Of instead of a CO2 getting absorbed by a plant, it could chemically combine with water, making H2CO3.
Most water, even distilled water has H2CO3 in it, ultra pure water will have much less of H2CO3 in it.