Aerospace engineer Robert Zubrin, in his 1996 book “The Case for Mars,” proposed the following time-table (ref) in a 2004 interview. The answer to OP is in #4:
RZ: If one considers the problem of terraforming Mars from the point of view of current technology, the scenario looks like this:
1. A century to settle Mars and create a substantial local industrial capability and population.
2. A half century producing fluorocarbon gases (like CF4) to warm the planet by ~10 C.
3. A half century for CO2 to outgas from the soil under the impetus of the fluorocarbon gases, thickening the atmosphere to 0.2 to 0.3 bar, and raising the planetary temperature a further 40 C. This will cause water to melt out of the permafrost, and rivers to flow and rain to fall. Radiation doses on the surface will also be greatly reduced. Under these conditions, with active human help, first photosynthetic microbes and then ever more complex plants could be spread over the planet, as they would be able to grow in the open. Humans on Mars in this stage would no longer need pressure suits, just oxygen masks, and very large domed cities could be built, as the domes would no longer need to contain pressure greater than the outside environment.
4. Over a period of about a thousand years, human-disseminated and harvested plants would be able to put ~150 mbar (millibars) of oxygen in the Martian atmosphere. Once this occurs, humans and other animals will be able to live on Mars in the open, and the world will become fully alive.
That’s the scenario, using current technological approaches. However technology is advancing, and 23rd Century humans will not conduct their projects using 21st Century means. They will use 23rd Century means and accomplish the job much faster than anyone today can suppose…