How soon can this happen?

98% of any habitat's mass will be in radiation shielding. Asteroid mining of water for rocket fuel is imminent, & the slag can be used for radiation shielding, including high transparency fused silica wall shielding. Growth from a seed structure to the mature habitat will require decades, but current progress developing asteroid mining is making the deployment of an expandable shielded rotating habitat look like a realizable goal. Click the link below for information on asteroid mining technology.