You're right about that, but I do think they will happen within the next 500 years. Several centuries may seem like a long time, but it isn't really. What we can do now to help move things along is to develop baseline technology so that a spacefaring civilization can begin to emerge. Once basic space infrastructure is put into place, construction of massive colonies won't be that farfetched.

Mass producing heavy lift launchers is the first step. Rockets capable of lifting 150-200 tons into orbit. The Aries V is a good step in that direction. I've heard that Aries V will cost around $180 million per launch. If the number of launches are increased, then the price will go down further.

A useful space station would be of great benefit as well. Something along the lines of the original dual keel design of the mid 1980s. Put it in a higher altitude 27 degree orbit so that it can be used as a stepping stone to the wider solar system. It's primary purpose would be to test technologies for living and working in space. Testing life support, energy production, waste management, propulsion, and refining space construction technologies, such as new welding techniques and material sciences, would be critical to further develop our ability to expand into deep space. A large station could also be used as a shipyard/homeport for deep space vessels.

The upcoming lunar outpost will also be useful in helping us develop the technologies for mining and refining space resources. There's no reason why the structural elements of a space colony can't be made out of lunar/asteroidal materials.

IMO, the amount of progess that's made over the next 50 years will determine how fast such colonies move from fantasy into the realm of reality.