Depends on what type of clamps - the issue is the pressure on the skin and tissues underneath, and the restriction of blood flow.
Anything that restricts blood flow is potentially damaging but you need to be realistic. Conventional clothes-pegs are unlikely to do any real damage no matter how long they are on. Clover clamps are much more serious as they are stronger and also localise the pressure into a small area. Then there are clamps using a screw thread to tighten them, and these could kill the tissues by stopping blood flow completely.
So as the liklihood of stopping blood flow rises, the time should shorten, but giving actual times is impossible as we are all different and we don't know the exact type of clamp.
I've experienced some very strong stainless steel imitations of stubby clothes-pegs for about 15 minutes. The nipples were white, on removal of the clamps, but recovered within a few minutes - well, the colour did, they were sensitive for a day or so. Then clover clamps, pulled under tension to tighten, pulled off, replaced and the cycle repeated a half-dozen times, very painful and took the surface skin off but recovered over a day or so.
One way of determining the safe time is to stop when you can no longer feel the clamp. First, you are getting no fun, and second, it suggests the nerves have stopped functioning, which implies a loss of blood-flow.
As a rough guide, I'd say an hour is too long, half an hour is probably OK for all but the most serious clamps. But it helps if you are a masochist
