I think you can adjust them, so you can change it to the level of pain you want. You don't want to leave them on for an extended period of time because they can cut off bloodflow, but they should be fine to leave on 15 minutes or so
The reviews say that they're adjustable, so that might help some. Do you have any experience with clamps or are you just looking into them? There's no need to be concerned about any damage being done to your nipples unless you're extremely aggressive with the clamps that are available here.
my nips are pretty use to pain since i nursed and all. i like being pinched a bit.. for me, there is no pain at all. but i would suggest starting with tweezer clamps. these compared to the press clamps... the press clamps hurt more.. the Spartacus adjustable clamps hurt more than these too. to me they feel similar to the magnetic clamps..