1. Take McKibben's 2nd number (565 Gt of CO2 as the global carbon budget to have 80% chance of staying under 2ºC) and let's call it the somewhat sane global carbon budget: SSGCB. It is only somewhat sane since a 2ºC rise is no walk in the park, but already represents straying into territory agreed by all world governments as representing "dangerous anthropogenic interference with the climate system".
By the way, 1 Gt equals one billion tonnes. Current global emissions are above 31 Gt each year - and rising. Also, remember that there is a difference between emissions and atmospheric concentrations.
2. In this list, find your country's current population and its proportion of global population.
3. Imagine that this percentage is your country's fair share of that total carbon budget. Admittedly, this ignores past emissions and so pretends that some countries haven't been hogging far more than their fair share for as long as we've known about this problem. But let's go with it for the moment to keep the picture from getting unmanageably complex and just remember that the results will be skewed in favour of those countries with a long history of fossil fuel use.
4. So multiply 565Gt CO2 by that percentage you found in step to find your country's actual share of the SSGCB. This is how many Gt of CO2 your country can emit between now and, well, pretty much forever, but let's just say 2050 for now, since that seems to be about as far ahead as any national government cares to think.
5. Now find your country's annual emissions in this list. It's a few years out of date, but it will do for the sake of this exercise. This is in thousands of tonnes (kt), so divide it by one million to convert to Gt. We're also going to assume, for the sake of simplicity, that national emissions are not changing (neither are populations). Since most countries' emissions (and populations) only change by a small percent each year, this isn't too much of a stretch for a ballpark exercise like this.
6. Now divide your country's actual share in the SSGCB (from step 4) by annual emissions (step 5) and see how many years it will take you to blow through your entire budget. After this point, all future emissions are stealing from everyone's else's right to emit.
For Australia, we'll be all done by late 2016.