If you're referring to the US and Canadian military plastic HDPE Scepter fuel cans (MFCs), I saw a US Army Logistics memo years that stated that MFC's were for "temporary fuel storage" only. It basically limited the military to storing fuel in MFC's for no longer than 6 months.
MFC requirements are contained in Commerical Item Description A-A-59592, “COMMERCIAL ITEM DESCRIPTION: CAN, FUEL, MILITARY: 20-LITER CAPACITY“
Permeability requirements in A-A-59592 are contained in para. 2.2, Operating Requirements, which basically says that the MFC will meet ASTM F852, “Standard Specification for Portable Gasoline Containers for Consumer Use”.
ASTM F852 states the following for permeability:
4.8.2 Permeability—This requirement applies only to nonmetallic
containers. The filled container shall not have a weight
loss greater than 1 % when tested in accordance with 8.6.
ASTM F852, para. 8.6 states:
8.6 Permeability Test—Fill the container to its rated capacity
with Reference Fuel B (70 % isooctane and 30 % toluene by
volume), as defined by Test Method D 471 and secure the
closures. Weigh the container accurately to the nearest 5 g
(0.01 lb). After storage for 30 days at 23 2°C (75 3.6°F),
reweigh the container and calculate the weight loss.
Bottom line: Steel container, properly sealed, don't allow diesel fuel vapors to leave the container. Plastic containers, like MFCs, do. How much? Don't know since I don't know the chemical differences between the chemicals mentioned above and diesel fuel nor the actual vapor loss through MFC plastics. I don't believe there is any issues with the MFC HDPE plastics breaking down over time.
Practically speaking: My bet is that anything less than 2 years is probably OK. I know I'm not dating my diesel fuel that's in my personal MFCs.
I hope this helps.
Merlin