I would follow the same rules that I would for any other body part. Swollen, red and purple while may be pretty to some, can cause short and long term damage.
Lack of blood flow to the tissue, can begin causing damage in as early as 15 minutes. That's why most folks use a 10 to 15 minute rule with any type of tight bondage.
Clothes pins, pegs or clamps on the tender pussy lips can be considered a form of tight bondage. At the very least, it restricts blood flow to the area.
If you're not sure, you can always become a test subject.
To discover what your wife might be able to handle, you can put a clothes pin or three on your own penis and balls. Get out the stop watch and see how long you can take it before you lose sensation in the area or begin to swell.
Then report back and let us know how it went.
What's good for the goose is good for the gander.