Not all clothespins are created equal.

On another board there was an extensive article "clothespins 101"

How tight or loose can be changed as well.. by either overstressing the spring or adding tension to it.

In any case, the max length of time left in one postion is not a set thing, rather should be determined by the each sub. As was stated above moving them with a break inbetween is the best course to take.

In our POV, I apply them and after the last one is on, start a small five minute timer, one of those like a hour glass, so its quiet but I can keep and eye on it. This becomes doubly important when they are placed in the much more sensative areas, other than the nipples. We have gotten up to 45 minutes with 5 minutes on 2 minutes off.. and sometimes.. those two minutes can be just as interesting