How long can you usually stand the nipple clamps. i can only have it on for 30min. there's once when i was chatting with my master, he said i can't take it off without permission, i had it on for 40min. otherwise, when i'm asked to have it on offline, the longest i can take is 30min. and often can't bear anymore after 20min. is there anyway to make me bear the clamps better?