Its a reach. For comparison, lets start a city on top of Mt Everest? Better air, better weather, lots easier to get to, and help is only days away instead of months. Yet we haven't begun to do things even that ambitious yet.
It's hard to say when and how, of course. It might not happen in any serious fashion until asteroid mining and space tourism became real industries, which should result in improved tech and much lower costs for building Mars colonies.
Interplanetary trade will be a huge business. Think of the old east India trading companies only bigger.
There is no spice on mars. Even if there was, it would not make sense to transport it to Earth[1]. It's not clear what would be the business model. The only thing I see is tourism : hotels, casinos, etc. So basically like Las Vegas, but on mars. Just as in Futurama [2].
Could be cool, but probably not exactly the romantic image Musk's supporters might have in mind.
1. "Well, I think any natural resource extraction on Mars would be - the output would be for Mars. It definitely wouldn't make sense to transport Mars stuff 200 million miles back to Earth. Honestly, if you had like crack-cocaine on Mars, in like prepackaged pallets, it still wouldn't make sense to transport it back here. It's be good times for the Martians, but not back here. Resources would be for a colony to use." http://shitelonsays.com/transcript/elon-musk-at-mits-aeroast...
Sounds like zero supply of something a colony might want to pay for. Trade goes both ways. I never took a guess at time frames this could happen in.