six months (I am guessing) which I noticed because I was replacing the springs with Swift ones. It was not on the springs though - just the tube body. I had to have a friend hold the shock body while I used a hammer on the locking nut wrenches to unbond the aluminum locking rings. I tightened them when I installed them with normal force on the rings - a few foot pounds of torque.
I figured it was more a galvanic reaction between the dissimilar metals than rust holding them together. Something I had not thought of when thinking about making on-car adjustments later. I removed any dirt and rust with a brass brush and coated the threads with PB Blaster to help prevent future rusting.