Hi Rob,
I also find the pause between the breaths lengthening. I try not to control it. I go with what feels right. When the breath slows down and oxygen consumption is lowered, the body compensates by pulling the sexual energies from the pelvis into the spine and towards the brain. This is in effect kundalini rising. I know that the slower that my breathing becomes, the more ecstatic conductivity increases. I have caught myself at times during meditation, not breathing and by noticing this breathing resumes.
As for the number of spinal breaths. My routine is to typically do 25 rounds. I use mala beads to keep track as opposed to a clock. Some people find mala beads a distraction. I always do a minimum of 25. This takes from 14 to 17 minutes depending on how slow the breath can naturally go. If I am pressed for time I may skip samayama or Yoni Mudra in exchange for the extended spinal breathing as I feel the most benefit from it. We don't want to cut short the time in the mantra though. Spinal breathing is a nervous system cleansing exersize. It is the cultivation aspect. Mantra helps integrate the process. It is allowing the garden to grow.
With Peace,
Paul
