Nipple clamps increase sensation in two main ways.
They increase sensation because they are clamped on, and therefore cause continuous pressure signals to be sent to your brain. This has the effect of making the nerve endings more sensitive to touch and pull because that decrease in blood supply. Tightness is easier to answer than time. It is generally safe to clamp skin tissue so that the clamp stays on, but doesn't break or cut into the skin, and doesn't cause bruising of the tissue underneath the clamp. A little can feel like a lot, so no need to put the clamp on the highest setting for a rewarding experience.