My immediate suggestion would be to make it a -30 modifier, ticking down at 1 point every 3 months.
Right now, there is only one number available to be modded (PEACE_TIME_MONTHS). In order to try out your suggestion, we would need a PEACE_TIME_RATE as well, to control how fast the initial value from PEACE_TIME_MONTHS decreases. It currently decreases at a fixed rate of 1 / month.