"Total venous occlusion for 10, 30, and 60 minutes showed a significant decrease in the number of flowing capillaries through 24-hour postreversal. There was a significant drop (p < 0.01) in the number of flowing capillaries from 30 minutes of total venous occlusion to 60 minutes of total venous occlusion at all times. These results indicate that irreversible tissue damage occurs in a very short time interval (60 minutes)"

Microsurgery and Hyperbaric Laboratory, Division of Plastic Surgery, University of Nevada School of Medicine, Las Vegas 89102, USA

basically what this says is, if you cut of the blood flow completely for 60 you WILL sustain permanent injury. now most bondage (clams included) will cut off a large amount amount of the blood flow but not all. The 15 minute rule is a good rule of thumb because as you see above it start damaging cells at 30 minutes, at 60, there dead and gone forever, but also they will form scar tissue, which if it doesn't attach or remain attached it could float around in your blood stream damaging your kidney's or even causing a stroke or heart attack.

now if you enjoy your nipple clamps for long periods of time great have a blast, but here is my 2 cents. put them on for 15-20 minutes and take them off, its going to cause that rush of blood flow and your cells are going to refill and its going to sting ever so nicely, after about a 5 minute "breather" put them back on. This give your cells the chance to breath and refresh. Kinda like swimming, you dive underwater and push your body to the limit as long as you hold your breath, then you come up for air, gasp a bit wait until your lungs refill nicely then you can take another big breath and go back down.

Please note, I am not a medical professional this is only an OPINION. I think anyone (ESPECIALLY TOPS) should take a few basic nursing classes at minimum personally I was (yea I let it lapse) a certified EMT. There health safety and well being is your responsibility.