Some of what I've read says that it can take as much as 6 months for the full benefit of plaquenil to be felt. I'm sure others on the Forum have more clinical info for you. My own experience is that I've been on it for about 5 months now, and I do feel a difference in my fatigue levels and some of the 'fog' that you talk about. It won't help with pain or swelling of your joints, if you suffer from that. But it can improve your general well-being. Hang in there. I'm just learning that it's a long, long road.
"If you trust Google more than you trust your doctor than maybe it's time to switch doctors."