If you do it right, it's painless and quick. In my experience cutting off the carotid arteries gives me the sensation of tingles all over my body and falling. It's fade to black from there, and then you're out. That takes only 10-30 seconds. The trouble is doing it right.
My advice is to not put pressure on gradually, it is NOT fun to cut off circulation to your jugular veins (which gives you the exploding head sensation because blood is flowing into your head, but not out) before cutting off the carotid arteries. However, arteries are higher pressure than veins. It takes less pressure to cut off the jugular than it does to cut off the carotid. If you apply gradual pressure, you'll have to endure that awful sensation.
So, the best approach is to tighten the noose and drop your weight all at once. Also, I think it's best if your spine is straight for a few reasons:
1. If you have to lean into the noose to get the pressure you need, it's way too easy to move out of that position while convulsing or unconscious.
2. In my experience it's way easier hit the sweet spot when in an upright position. I'm not entirely sure why, maybe my circulation just sucks, but partial while standing or sitting with a straight back is best.
3. Leaning into the noose doesn't apply pressure effectively, it's uncomfortable, and you're less likely to get the sweet spot doing that. It's better if your noose is short enough that you don't have to lean, and instead just use your body weight.
You can practice with an unanchored noose to get a better idea of what's comfortable for you. For me, the higher the rope is, the better. The worst spot is the middle of the neck (where the adam's apple would be). Some people say below the adam's apple is better though.
All of that said, I'm in the same boat as the guy whose survival instincts kicked in. I tried to do full suspension recently but my noose was too low and I ended up doing partial. I convulsed like crazy and then suddenly I was in bed. I have no memory of how I got out of the noose. I'm guessing I was able to reach the stool in the few seconds I had before I would have blacked out. I'm not afraid of death but my instincts sure are.