Figure a loss of up to .5 octane per month. 20 months=10 octane points of loss which would bring it down to around 77 octane. Its not an exact science for octane loss.
With that much octane loss you would still need drain off enough to add 55 gallons of 114 octane at $8-10 a gallon.
IMO, you would be better off to start draining it all and using it in your cars in a 50/50 blend with fresh gas.