Breast bondage

My boyfriend is trying to persuade me into breast and nipple bondage. We already tried some moderate bdsm practices as a spanking or butt plug wearing but I am not sure if this practice is fully safe. He wants to try some rubber bands for my breast and nipples. I have D cups and bigger nipples. Although I watched some videos, I have no clue how tightly banding is still safe or how long my breast can be tied, etc.