I think you achieve max spell duration at 20% guilds.
So here are my questions:
1. If you're a mystic, this means 10% guilds will achieve max spell duration?
2. Does BE affect this? eg. at 80% BE I would need 25% guilds to get the max spell duration? Likewise, at 120% BE I could use only 16.7% guilds for max spell duration?
3. Does BE affect wizzie production? (I know mystic does). According to the wiki: "Building Efficiency affects all Flat Rate and Percentage-Based buildings." and guilds are a flat-rate building.