May I suggest a rotation period somewhere between 1-3 weeks for the Void? Where on that scale depends on how many different enemies are being added to the rotation, but the longer period can help with:
- Users that need time to plan out and prepare for the boss fight, through experimenting to see how each boss works or fetching gear specific for a boss.
- People that may be otherwise engaged for short periods of time; being busy for 2 weeks straight is a lot less likely than being busy for 3 days (less than a work week).
- Maintaining engagement with the game. This one can be 50/50, as people could be forced to wait for (period * number of void mobs on rotation) before they can fight the one they want, but on the other side it can act as a mid-term goal, sitting between seasonal (yearly) things and the weekly updates.
If you want players to wait a maximum of 3 months for a given boss, and there are 12 bosses on rotation, you'd have a weekly rotation
If you want players to wait a maximum of 3 months for a given boss, and there are 6 bosses in rotation, you'd have a bi-weekly rotation
This all depends on the number of bosses you plan to have on rotation at a given time, and how long you'd expect players to wait before they can take on a specific boss. This can also allow for "rare" bosses by having some number of time slots also rotate e.g.:
Every week there's a different boss, but on the 6th week (before it loops back around and starts again), the void fight could be chosen from a pool of bosses. Say N is a normal boss on rotation, and R1, R2 are 2 rare bosses. The rotation would look like so: N N N N N R1 N N N N N R2 N N N N N R1 etc.
Just some suggestions on how it could work. Also special event bosses like the May 4th fight could cause issues such as cutting a monster's availability short, so that's something to consider.