Due to the speed of transiting Mars, this occurs about every two years, and for the most part will last 2 to 3 months. Should Mars turn retrograde, it can take 6 or 7 months to move through the 12th house.
What I’ve observed with myself is that everything seems to slow down, almost put on hold, and often I’ve found that a sense of “self-undoing” can seem right around the corner! In my own horoscope, Mars is at the AP (00 Cancer) and rules the 12th house. Perhaps that contributes to what I’ve experienced each time tr. Mars visits its home stomping grounds, if you will.
Granted there will be other more significant transits and Solar Arcs occurring simultaneously. For example, this year when Mars was moving through the 12th, I also experienced the retrograde hit of tr. Neptune conjunct Mercury, the last hit of tr. Saturn opposed Mercury, and the final hit of tr. Saturn conjunct Uranus. With Mars moving through the 12th, it seems a bit grimmer!
Two years ago in 2005, Mars went retrograde through my 12th house for an extended period of 7 months. At that time the larger measurements were SA Saturn=Pluto, SA Pluto=Saturn, and tr. Pluto opposed the Moon.
Granted the background measurements may not form a pretty picture, but it just seems to me that tr. Mars through the 12th house has a void-of-course Moon feel to it.