After 4 years that link does not work anymore. Pity.
I usually guage it by the colour and temperature of the breast to determine if it is time for some blood supply to them. (Some people believe 20mins max. I think it is slightly more personal than that)
Colour:
Red=goody
Purply=still yummy
Angry purple=stop now and restore blood supply.
Black=stop immediately.

Temperature:
If it is cold to the touch it means it needs blood supply. Stop.

Nothing stops You from reapplying the pegs. After the colour has been restored.....(Now that was an evil thought!)
One needs to remember that the nipple and most especially the areola is designed to carry some heavy abuse, the tissue around that area, the breast, while pretty resillient, is not. I know masochists with pretty serious lumps after hard tit torture...so, beware.

That said....nipple play is delicious...

Pegs, ice, hot stones, wax, clamps, weights, biting, chewing, sucking, licking......oh my, is it me or is it suddenly quite hot in here?