No partisan activity doesn't decline over time.
It does in Doomsday and Armageddon, though.
It represents the chance of revolt in the province.. when there is a revolt in a province, a partisan militia division takes over control of the province and if not dealt with they will move to other provinces as well.
I guess the OP figured that much. He meant what the 24% refers to.
While I don't know the answer, I guess it's something like that:
Each day each province runs a check whether Partisans revolt or not. The percentage chance is obviously much lower than 24% a day. So like with almost all other values in the game, Paradox might have decided to give a cumulated percentage per month or per year instead, as you can then actually see something.
Imagine the daily chance was displayed: 0.03 percent. Would any player care to commit precious manpower to suppression then?