Suggest you contact either Baker-Hughes or Nalco as they can better direct you. The stability question involves, among other things, the fuel itself and any stability additives added, exposure to oxygen (oxidation of fuel), time length of storage, etc.
I saw a show on TV about one of the new air craft carriers that said it was fueled up when it left the dock and wouldn't be fueled again for 50 years (maybe 25). I have no idea if there is something special done to keep the fuel fresh, but that is a pretty long shelf life.
I know that's not very helpful, but I thought it was interesting.
It doesn't talk specifically about re-fueling the fuel tanks (diesel and/or jet?), but I found where it said the reactor fuel should be good for 20 years without re-fueling.
On the show I watched I thought it said it didn't need to come in for any fuel in that 20 years, but you are probably right that they were only talking about the nuclear fuel since 3 million gallons is only 410 gal/day which doesn't seem like much when you're burning it in airplanes.
A question on the OP. How does the cost of running the generator compare to buying electricity? Would it be cost effective to just burn up the diesel once in a while to keep it fresh?
As stated above it depends on many variables. The time can be months to a year or better, but we have seen deterioration normally starting a about two months on a fleet of shrimp boats.
We use PRI-D in all our fuel tanks with excellent results. Our main reason is that the boat captains will not run the oldest fuel first and some fuel could be year old.
Depends on how it is stored, the criticality of demand and your suitability for use specification, which should be relative to your criticality of use. Speaking generally towards a use lacking criticality and high purity standards, in a tank vented to open atmosphere don't count on more than 6 months to 1 year. Stored under a dry N2 blanket with biocides and filtering once yearly, it could go as long as 5 years, or longer. In a highly critical military emergency defense underground storage plant with high purity standards, blanketing, biocides and filtering countermeasures, we assumed a 2 year use and replacement schedule would be conservative.
That is not to say you couldn't burn the stuff after 6 months to 1 year, simply that your pre-use filters may need frequent replacement, you may have excessive bios, dirt, water, etc. in there, or your engine performance may suffer. That's why its essential to match your storage system reqirements to the criticality of your intended use.
Our rule of thumb is 6 months, with no "special" storage facilities.
Since you are using it for a standby generator, I assume that you have some maintenance on the generator as well.
Run the generator once a month for eight hours to verify it is in top condition when needed.
Keep the tank full, this prevents getting to much moisture (condensing) in.