Once FMM starts the Volt will start to burn all of the fuel in the tank, i.e. 5 gallons. You can reduce the amount of fuel that will be consumed by FMM right now by mixing fresh fuel with the old fuel. The Volt will compute the average age of the fuel ((age of old fuel * gallons of old fuel in tank) + (age of fresh fuel * amount of fresh fuel added))/2.Question: How much fuel will be burned off, all 5 gallons, a percentage, how much?
It all depends on how much gas you are comfortable having sit in the fuel tank. If you allow FMM to use up 2.5 gallons of old fuel, starting now, and then purchase 2.5 gallons of fresh fuel, FMM will be suspended with 5 gallons of a mixture of 50% old fuel and 50% fresh fuel in the tank. In six months the Volt will ask you to allow FMM to run again.
You could suspend FMM right now by filling the fuel tank with 4 - 5 gallons of fresh fuel but then you would be carrying a full tank of fuel in the Volt when you regularly need to use only minimal amounts of gas. In this case too, in six months, the Volt will ask you to allow FMM to run again.
Since you normally use so little gas you may want to allow FMM to burn up all of the fuel in the tank (get the amount of old fuel in the tank down to less than 1/3 or 1/4 gallon) and then add 1.5 gallons of fresh fuel. That amount of fuel should last about a year with Engine Maintenance Mode running for about 10-15 minutes once every 6 weeks. Then, a year from now, you would purchase another 1.5 gallons of fresh fuel.
I keep my Volt's fuel tank between 1/3 and 1/2 full because I like knowing I have 125 - 200 miles of ICE range. I don't wait for EMM to run once every six weeks. I use Hold mode and run the ICE engine for about 8 - 10 miles every two weeks to circulate the fluids and and to fully heat up the engine oil to 190 F + to drive out any moisture from the engine.