that would roughly equal 900mio usd. to archieve that, the value of all bitcoins combnied has to increase just 60% per year. i think thats a very conservative goal. bitcoin passed 1mio usd just eight month ago, right now its over 90mio. even though a slowdown in growth is realistic, imho that much of a slowdown is not.
The growt of this first 2 years is somehow "doped" by speculations. When a strongest economy will grow there will be a faster exchange of money (more velocity in economics terms) so we can reach a "GDP" maybe 3-4 times the number I've depicted but the M2 will stay below 1,000mio $: if 1Mio people (20 to 40 times how many use it now) will using bitcoin in 5 years that means 1,000$ each with a GDP/per capita of 3/4,000$; actually there are at least 100 nations on the CIA facts that don't reachs a similar GDP/per capita.