KUMBHAKA: this is the time, theoretically speaking, the moment when there is mental stillness, this is the moment when the god is present!
The whole phase of inhaling and exhaling only strengthens on the inhale, disappears on the exhale! There is appearance and disappearance of the image.
In KUMBHAKA, the image remains!!!
We use MANTRA to count time in PRANAYAMA
For example,
in STAMBHA VRITHI, the same MANTRA is used thus:
PURAKA (inhale) = 1 MANTRA
ANTAH KUMBHAKA (retention after inspiration) = 1 MANTRA
RECHAKA (exhale) = 1 MANTRA
BAHYA KUMBHAKA (retention after exhalation) = 1 MANTRA