Used to change it up every 4 weeks. Over time I've felt I could have milked a bit more out of each program if I stuck with it longer, so now I go more by feel. Some exercises I feel work the muscle perfectly (mind muscle connection, good contraction and stretch, smooth resistance curve), so I never swap them out. Other exercises I struggle to isolate the muscle I'm trying to target, so I'll swap them out after a few weeks.