It will all depend on what she likes & can tolerate.
We've never tried the Butterfly (there's another name for them that escapes me) style clamps, but do enjoy the alligator clamps (protective plastic sleeves removed) with the adjustable screws.
Placement is important. Attaching them to the base of the nipple can cause pain/pleasure, but putting them on or near the tip of the nipple can generate a more hot, burning sensation. When to pull on them, how hard, how often, etc., will depend on how she reacts.
One thing to keep in mind is the burning sensation is more intense after orgasming. Not to be redundant, but how long to leave them on will depend on what she's comfortable with.
Practice & experimentation are the best ways to find the answers to your questions.
Oh yes, have fun
!