Yep, I heard 15 - 20 minutes for 'proper clamps'. Not sure about lighter things like lcothes pegs but I would follow the same rule.

You can, however, take them off, massage some blood back into the nipples and put them on again.

The important thing is to not stop the circulation to the area clamped for too long otherwise you risk ischeamia (tissue injury caused by lack of oxygen). paramedics/firefighters use the 15 minute rule to decide whether or not to release someone from fallen rubble (if it is less than 15 minutes then the tissue will be ok so you get them out asap, if it is more than 15 minutes do nothing until a more qualified person can look at them as removing the restricting object could kill them).

If the nipples change colour to anything other than red or white take the clamps off imediately no matter how long they have been on.