The effective dose limits for a nuclear energy worker is set at 50 mSv in any one yearSo this is 50 milli Sievert per year... or 50/365 = 137 micro Sievert / day or 137/24 = 5 uSv/hr.
Normal background around my place in Quebec, north of Montreal is 0.50 uSv/hr.
So this limit is 10x the normal background I get.
But to compare with what you get in a plane... let's take as an example the short trip to Toronto I took... http://radio-activity-studies.blogspot.ca/2013/01/radiation-durant-un-voyage-en-avion.html
While cruising at 35,000 feet you get exposed to 3.5uSv/hr or 7x the background.
So to get to 50,000 uSv (50mSv), you would need to spend 595 days in the air at the elevation.
So the numbers 321 days is a bit low, but that depends on the background, elevation and which regulation they used to calculate. The more elevation you get, the higher the radiation dose you get.
Bottom line, flying is safer than working in a nuclear power plant (if exposed to the limit, which never happens)