Most of the time I don’t look at the clock at all; I clamp until it feels right to take the clamp off, and repeat until I feel like I’m not going to get any more benefit from another set.
Learn to really listen to your body, then go by feel.
Do you feel that you are less sensitive to your dick when it’s in a clamped state? For example, can you tell that it’s cold without touching it? I can tell if it’s not in a clamped state but I have a problem telling if it is. I feel that for me when it’s in a clamped state, I get less sensory input from it, as if less sensory data is being sent out. Almost like that idea that when you are in pain, holding your breath may ease the pain a bit.