From GlobalDefines.xml :
The modifier BUILDING_PRODUCTION_DECAY_TIME has a value of 50, probably meaning that building production starts decaying after 50 turns.
The modifier BUILDING_PRODUCTION_DECAY_PERCENT has a value of 99, probably meaning that building production decays at 1% per turn.
The modifier UNIT_PRODUCTION_DECAY_TIME has a value of 10, probably meaning that unit production starts decaying after 10 turns.
The modifier UNIT_PRODUCTION_DECAY_PERCENT has a value of 98, probably meaning that unit production decays at 2% per turn.
I don't know if the decay rate is taken from total production cost of the building/unit or from already invested hammers. But that could be tested easily if someone wants to know.
I also don't know if these modifiers are adjusted at the different speed settings (epic, normal, etc.).