If you teleported an earth-like atmosphere to Mars, it would last somewhere in the neighborhood of upper thousands, low millions of years. That would give us a

If you teleported an earth-like atmosphere to Mars, it would last somewhere in the neighborhood of upper thousands, low millions of years. That would give us a good chunk of time to figure out the whole "create a magnetosphere" problem. Of course we don't get to just magic an atmosphere out of thin air, but there are a few more realistic proposals.