According to my speed model, the growth rate of consumption is approximately double the growth rate of population. That is, if the population increases by 1% per year, then the amount of mass consumed will increase by 2% per year. This is perhaps the most important practical constraint on the future, since we must locate and be able to distribute enough resources to produce the new members of the population and for them to consume.
Without fundamental changes to the ways we acquire and distribute resources, I project that the population is unlikely to grow beyond about eight billion people. If we do make such changes, our daily consumption must be able to grow by about a billion pounds per year for each percentage point of annual population growth (or half that for each percentage point of total annual consumption growth).
Since the biosphere is too stressed to provide more resources and services (we are currently consuming the biosphere itself to make up the difference between what we use and what it can give us), we must pay for our growth by effectively “eating rocks” on this planet, in space, or both. While mining already provides most of the materials and energy we use, we will need to begin converting what we mine into food and water (desalinating the oceans and piping water inland could work for a long while, though the side effects on climate will be a problem). We will also have to embrace our role as exterminators on a grand scale, converting mass (living and not) into human biomass and artifacts as fast as we can.
If we go into space, the Moon is the best immediate target for mining. It falls within the required transit time, but there are huge technical challenges involved in moving a billion pounds daily between there and here (never mind safety: this is the equivalent of a 300 foot-wide asteroid).