For me, the drop-off is almost immediate. During the early days of my honeymoon, (as opposed to now, where I’ taking 1U a day) if I exercised with any insulin on board, after 20 minutes of cardio, I’d routinely be in the 60s. Even now, exercise can knock an extra 10-15 points off my sugars. I believe, but am not certain, that the effect lasts for several hours after exercise.
