Elon Musk thinks that it would take between 40 and 100 years for humanity to go from landing a ship full of colonists on Mars to establishing a self-sustaining civilization. At his company SpaceX's event today, Musk laid out how a fleet of ships that could carry a minimum of 100 people, flying every two years, could populate a Martian city:
If we say the minimum threshold for a self-sustaining city on Mars, or civilization, would be a million people, and you can only go every two years, if you have 100 people per ship, that's 10,000 trips. So I think at least 100 people per trip is the right order of magnitude, and I think we actually may end up expanding the crew section and ultimately taking more like 200 or more people per flight in order to reduce the cost per person.
But 10,000 flights is a lot of flights. So you'd really want ultimately I think on the order of 1,000 ships. It would take a while to build up to a thousand ships, and so I think if you say when we reach that million-person threshold, from the point at which the first ship goes to Mars, it's probably somewhere between 20 to 50 total Mars rendezvous. So it's probably somewhere between maybe 40 to 100 years to achieve a fully self-sustaining civilization on Mars.
A few hours ago, Musk announced a new "Interplanetary Transport System" that's theoretically capable of sending humans to Mars at a cost of around $200,000 a person, instead of billions of dollars. Of course, all of SpaceX's plans involve a lot of speculation — and as Musk's statement above shows, they will take many decades to come to fruition, even if everything goes right.