I'm not sure what the exact cutoff is. Right now I've got 2147 on my BTN/MIN mining retainers and 1742 on my FSH retainers. That's getting me max yield on everything except level 80 Unspoiled and Folklore items (not worth upgrading their gear for those). If you haven't added Guerdon materia to their current gear, putting some on should get you to max yield quickly.