How long can I expect to stack pirate massacre missions in a source system, given that the faction states change periodically?

After more than a thousand hours of Elite Dangerous, I've finally gave stacking pirate massacre missions a try this week. And boy, was it fun. I really enjoyed it, especially because of two very useful tools developed by fellow commanders from our community:

• PVE tool by u/cold-n-sour (VicTic)
• OD Massacre Stack Manager by /u/warmedxmints

However, as I was improving my reputation with the local factions at 36 Persei, I suddenly stopped getting pirate massacre missions on the board. I went check for the faction states and realized none of them were under Pirate Attack anymore.

My questions are:

1. How long should I expect to get a decent flow of massacre mission in a source system given that the system and faction states change periodically? Is it just a week, or can I manage for it to be longer?
2. What would be a strategy to keep the reputation I've built with the factions at the source system in the long term for stacking pirate massacre missions?

I'm off to find a new source system with the correct states for pirate massacre missions (war, civil war or pirate attack, what else?), but I'd like to know if the work I put towards a given source system is only bound to last a week.

Thanks!!