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Here was clarified how but not how long:

How will the SpaceX ITS return from Mars?

Because, for that time in case everybody wants to return you need to support survival on Mars (the ship is stated to carry up to 100 people).

Probably already the very first mission makes sense to found a colony unless there is a nice tourist round-trip.

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  • $\begingroup$ PArt of the equation is how much power will they have to run the Sabatier and electrolysis plants. $\endgroup$ – geoffc May 4 '18 at 12:37
  • $\begingroup$ if they would be able to multi-use ship solar modules, their max. power shall be up to 200 kWt but this needs to subtract electricity required for life support. Unless cargo has further solar modules or other power sources like en.wikipedia.org/wiki/Atomic_battery $\endgroup$ – J. Doe May 4 '18 at 12:44
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From Tom Muller (CTO of Propulsion at SpaceX), the present plan is to power ISRU efforts with solar. He says "If you try to do it with solar; it’s extremely difficult, but doable. To get one ship back, you need about eight football fields worth of solar cells on Mars."

To me that reads like increasing the surface area could speed the process; it just sets one edge of the envelope at two years per ship. He also goes on to say "so that’s tricky. It’s much better to use nuclear, fission reactor, it gets, you know, more compact; you actually get more; you get more power out per pound of reactor than you do out of solar cells, so it’s more mass-efficient. So if you’re taking it to Mars, it’s more efficient to ship reactors than it is to ship solar; it’s just that nobody’s really developed a space reactor yet. We’re working with NASA on that, and hopefully they’ll get funding to develop that. They’ve got a program called kilopower going that’s like, ten thousand watts, a 10 kilowatt reactor. We need a megawatt, but you know, you need to start somewhere."

With nuclear power available the process could likely be sped up, but there are many political and regulatory challenges to that. Early flights may depend on solar, and take most of the two years between launch windows to produce fuel.

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