It is more of a guesstimate as there are many ideas and suggestions,
some of which have not been tested and so whether they would work is
another matter. It is suggested that could take from several centuries
to 100,000 years.
In 2000 NASA sponsored a conference on ?The Physics and Biology of
Making Mars Habitable? at the Ames Research Centre. By way of
introduction, this is their web site on the subject.
This page briefly details the conference proceedings (originally on
the Massachusetts Institute of Technology web site, but now only
available through the internet archive).
This site in turn, links to this site called ?Terraforming Information
Pages?. As it is a referred site from the conference I assume it is a
reliable source. The site contains several pages of information.
These are some general articles on the subject.
Finally, if you are interested, these papers are available for purchase.
The Physics, Biology, and Environmental Ethics of Making Mars Habitable
Mar 2001, Vol. 1, No. 1: 89-109
?If a planet-wide martian biosphere converted carbon dioxide into
oxygen with an average efficiency equal to that for Earth's biosphere,
it would take >100,000 years to create Earth-like oxygen levels.?
The Biological Terraforming of Mars: Planetary Ecosynthesis as
Ecological Succession on a Global Scale
Jun 2004, Vol. 4, No. 2: 168-195
The Future of Space Exploration
Bringing Life to Mars; The Future of Space Exploration; Scientific
American Presents; by McKay;
I hope this answers your question. If it does not, or the answer is
unclear, then please ask for clarification of this research before
rating the answer. I shall respond to the clarification request as
soon as I receive it.
Initial terms: mars atmosphere terraform , led me onto the above sources.