A unit that is decaying is aged 5 years per combat round and has a variable chance to take damage1). This is doubled for units that have the effects of quickness. There is some chance for this effect to end every turn, however it is not well understood. An affected unit has a 1% chance to make a magic resistance check vs 15 every round of combat to be free of the decay2).
Like poison and profuse bleeding, decay will continue to apply its effects to units in the 20 turns following completed combat. It is unclear whether or not the chance for the effect to end still applies during this time.
A unit with the Unaging bless does not take the damage per turn and the aging is slowed down to about 1 year per turn.
Slowness halves the effect? Reduce by +2 or +3 years per turn (maybe it alternates or randomizes)?
Understand the mechanics behind why it sometimes does 1 every 2 rounds and sometimes a lot more - to old age units?
Better understand the mechanics behind decay ending
Combat usually doesn't last long enough for a unit to die from Decay. Decay is often employed to ruin a unit in the long run. Unless the unit has a long Life span (ei. many true giants, Elfs, Demons, Undead) they will suffer Old Age (Decreased stats, Higher Encomprence, become susceptible to become diseased during the winter).