Updated: The old terraforming question - Part 2
But wait, there’s MORE! We didn’t just stop producing interplanetary rockets when we started moving stuff from Earth and Venus! We kept producing more of these suckers so that we could start snagging water from comets and other (solar system relative to Mars) local sources. The more water we can get down to the surface, the faster and further our extremophile plants can spread and increase the speed we can produce O2 to be more breathable in around 60+ (90-100+ from present day) years (ballpark, depends on water availability and growing cycles, might still need masks to filter high CO2). At first, a large amount of the water will be held in the plants themselves (the ‘Green’ Mars phase). Over time (and if we continue building even more spaceships) we can start pulling hydrogen and water from the outer gas giants (Jupiter/Saturn’s gravity is a bit too hefty for harvesting unless we go for really REALLY big rockets), moons (several for water or the hydrocarbons on Titan) and comets (if we want to go really far out, or snag a few as they come further in toward the sun).
Depending on the speed with which we are continuing to produce spaceships, we could be moving thousands of tons of condensed hydrogen or water to Mars with even more ships being produced and hauling as time goes on. Each round trip would take about 1/2 - 3 years depending on orbits, how much we’re hauling, and where we choose to move stuff from. Hydrogen harvesting from outer gas giants directly and chemical reduction with CO2 to make water might cut 20 or more years from balancing out the atmosphere and creating seas, since it would be more efficient pound for pound of hauling mass (hydrogen is only 1/9th (or 0.111…) the total mass of water). We can in theory also get H2S (hydrogen sulfide) or acids like H2SO4 (sulfuric acid), HCl (hydrochloric acid), and HF (hydrofluoric acid) from the Venus atmosphere  as another hydrogen source, but they’re heavy, corrosive and poisonous. Extraction of sulfur compounds and acids might be useful if we’re trying to terraform Venus at the same time though.
All of this stuff heading to Mars brings up an interesting question: How could we streamline getting stuff down to the surface and keep our spaceships going as fast as we can? Space Elevators on Olympus Mons (big mountain on Mars) and Skyhooks (orbital tethers to let things down gently from space) ! The ideas that have been frustrating engineers on Earth for half a century and couldn’t work with current materials would be much easier with the 38% Earth-gravity of Mars. High quality carbon nanotubes could handle these jobs without too much problem (or Zylon or even Kevlar), and you could catch a potato-moon for ballast (probably Deimos since it is smaller, would need to move Phobos further out to be out of the way, and Deimos could possibly be used in material manufacturing of the elevator cabling since it has a high carbon content) on the other end after tying some of our fun VASIMR engines (or even normal chemical rockets) to it and moving it to geostationary orbit. Give an additional 20 years for setup, moving things into position, manufacture, safety testing and streamlining before the elevator or skyhook would be in frequent use, so about 60 years from present.
Now, I know what some of you might be thinking: Aren’t we just going to lose this nice atmosphere after building it up from the lack of planetary magnetic field? First, if the atmosphere takes a hundred thousand years to lose 1% of pressure, it won’t be an “immediate” issue. We don’t know exactly how fast it will be lost, but it’s certainly not going to all blow away overnight when there’s (relatively) still so much left after a billion years. Second, ever heard of MRI machines ? Producing magnetic fields isn’t exactly rocket science, and the Earth can be doing more than just making SF6 for the atmosphere warming, LENR generators, Mars-plants and rockets. Existing superconductors (expensive, hard to mass produce, would take a while)  can create massive magnetic fields given enough power (for power, see above) and cooled (space is pretty cold already though), or we could go with passive neodymium magnets to create an Earthlike magnetic field . All it would take is pretty much all the production of neodymium magnets for the next 75 or so years to produce enough to circle the planet a few times with then distribute them in arcs with magnetic fields oriented North-South in orbit around the equator. Sorry if you want some headphones or speakers in the meantime. My point is, a planet-scale magnetic field is completely doable in the longer term without something as extreme as asteroid collisions or moving Ceres into orbit around Mars to restart a planetary core dynamo like some people suggest . This is if after further study we decide we need a planetary magnetic field at all . Or we could just keep adding what gets lost. A lot less permanent, but might be cheaper that way in the short term.
So there you have it. Atmosphere keeps getting added and is around high mountain elevation pressures here on Earth (Mount Everest only has around 330 millibar, other tall mountains have higher than that)  at low elevations on Mars circa 100 years from present. Still kinda cold on average, but we’d have cold-and-tundra type plants/forests growing and a climate better than Antarctica for people to live in. When money is no object and the whole world works toward it, Mars is terraformed and (mostly) atmosphere-stabilized using only present-day and very-near-future tech in 100 years(for the breathable atmosphere) to 150 years (for the lakes, seas and even thicker atmosphere). Just think how much faster and more efficiently we can do this stuff if we’re still advancing science in the mean time?
References and Sources:
 Sputnik - 1957:
 Nuclear power:
 LENR accepted patent:
 Sulfur hexafluoride:
 Atmosphere of Mars (present):
 Atmosphere of Venus (present):
 Mars Space Elevator (Second answer for materials analysis):
 Superconducting magnets:
 Neodymium magnets:
 More extreme production of Mars magnetic fields:
 Water at Mars ice caps:
 CO2 and thickening atmosphere:
 Importance of magnetic field:
 Mount Everest: