Ran all stages in order — 5 months total, 2 months on ST4. It works and the change is permanent, although it could fade if you don't keep up the activities that utilize your brain and create new connections.
My one piece of advice: don't run an intellectual sub without a direction in your life, or it could channel into restlessness or mind chatter. When you have a direction, it is perfect. I'd even recommend stacking it with Emperor, Stark, or Khan — they help give the intelligence a shape.
I also stacked it with RoM, and learning skyrocketed into such heights — I was consuming and internalizing and filtering about 200–300 pages plus 4 hours of documentary in a day.
If you have a direction, you will greatly benefit from intelligence subs.