There are different ways to do the Ham Sah pranayama, and they will have different effects.
In each case, the exhalation is shorter than the inhalation, but "how short" can vary. In some methods, we expel the breath very quickly, almost abruptly, with a slight pressure to it--like a burst. Other methods are more relaxed. Personally, I've found that the method with very short bursts tends to energize me, while the more relaxed methods are better for meditation (it's easier to sit still afterwards).
The exact timing of each part of the pranayama will differ depending upon the person doing it. You should find a rhythm that works for you. Personally, I've noticed in doing them with a group that each person is going at a different pace.
As for how much of your breath to exhale, I would exhale as much as you can, comfortably. You're never going to expel
all the air from your lungs (I believe it's physically impossible), but you should exhale enough to be able to take your next inhalation comfortably and deeply.
As for the retention, I retain the breath only after the inhalation (i.e. I hold it in for a bit before I exhale it). During this retention, you should be in prayer, and your mind and heart should be focused on the Divine Mother.
Also, if you are doing pranayamas, you might find this audio lecture useful. I highly recommend it:
http://gnosticradio.org/lectures/lectures-by-topic/meditation/267-the-fuel-for-spiritual-experience-2-pranayama/view-details