Humanity was earth bound until Yuri Gagarin made into space. The first ventures into living in space were with Skylab and the Salyut space stations, though none of them were particularly successful at being serious human habitation in space. It was Mir that was really the first time humans lived in space
From February 5th 1987 to August 28th 1999 Mir had up to three people living space. Four men stayed more than a year, Valeri Polyakov for 438 days, Sergei Avdeyev for 380 days, and Vladamir Titov and Musa Manarov who both were on board for 366 days. In total Mir had 44 residents over 12 and half years.
If Mir had been in use full time for only an extra few months we would have had continuous human habitation in space since 1986, but there was about 12 months between Mir and ISS when no one was living in space
Since October 31st 2000 there has been someone on board the International Space Station at all times. From October 31st 2000 to September 18th 2006 there were three at least two people in the ISS at all times with the total rising to four when the replacement crew came up. We are coming up on 13 years of continuous human habitation in space.
Since April 2 2010 the norm has been for six people to be living on board the ISS at any given time. During the transition between crews the on board population drops to three for a two to four weeks. The current plans call for expeditions through to at least 2015 and most likely 2020 and possibly for ten more years after that. By the time the ISS is done, there will be more space stations in operation and more people living in space.
The Russians are already looking at what will replace the ISS. Some of the Russian parts of the ISS were constructed with the intent to be part of the next station.
The Chinese have started down the path towards their own space station with the launch of Tiagong 1 in 2011 and Tiagong 2 in 2015. They hope to reach something of the scale of Mir by 2020
Bigelow Aerospace is currently looking at a 2016 launch date for their first commercial space station.
Realistically by 2020 we are likely to see three to four space stations in orbit with a population of between 10 and 25 people living in space at all times. I know that this still does not sound like a lot, but it is a dramatic step up from where we are now.
Within 10 years it is quite possible we will see eight to ten space stations and a full time resident population of 80 people in space. It is at around this point that I think we may see the first person move to space more or less permanently.