What Mars governs
Mars covers assertion, physical energy, competition, anger and the practical act of starting something. In traditional medical astrology it also takes fevers, cuts and surgery. The useful question when reading it is not whether someone is aggressive but what form their effort takes and what conditions make them stop.
The sign describes method. Mars in Taurus applies steady force and will not be hurried; Mars in Gemini argues; Mars in Cancer withdraws and acts indirectly. All three are capable of considerable persistence, and none of them looks like the others in a disagreement.
Six weeks to a sign
Mars takes roughly two years to circle the zodiac and spends about six weeks in each sign, though the figure stretches when a retrograde is involved and it can sit in one sign for six months or longer. That speed keeps it personal. People born within a couple of months share it, which is a much smaller group than any of the outer planets gather.
Because the duration varies so much, the sign cannot be estimated reliably from a birth year. It has to be calculated.
Rulership and dignity
Mars rules Aries, and in traditional practice it also rules Scorpio, which modern astrology assigns to Pluto instead. Both attributions are in use, and the traditional one is worth keeping because it explains features of Scorpio placements that Pluto does not.
It is exalted in Capricorn, where effort meets structure and produces sustained work. It is in detriment in Libra and Taurus and in fall in Cancer. The fall in Cancer describes force that will not be expressed directly and therefore comes out sideways, which is a specific difficulty rather than a general weakness.
Retrograde every two years
Mars turns retrograde for about two months roughly every two years, so it is retrograde a smaller share of the time than any of the inner planets and only about one birth in fifteen or so falls during one. That makes a natal Mars retrograde genuinely uncommon.
It is read as effort that is directed inward or delayed before it is released, and often as a person who is slow to start and difficult to stop once committed. It is not a sign of weakness, and it says nothing about physical capability.
The house, and what a time changes
The house shows where the effort goes and where the friction turns up. Mars in the sixth house pushes at work and health routines; in the twelfth it operates out of view and is often invisible to the person themselves; in the tenth it is publicly competitive. Only a birth time settles which of these applies.
Mars is also one of the placements where the difference between two adjacent houses is stark, so an approximate time is not good enough here.
Questions people ask about this
How long does Mars spend in a sign?
About six weeks in normal motion, but a retrograde period can extend it to six months or more in the same sign.
Does Mars rule Scorpio?
In traditional astrology yes, alongside Aries. Modern practice gives Scorpio to Pluto, and both readings are in use.
How common is a natal Mars retrograde?
It is retrograde for about two months every two years, so roughly one birth in fifteen.
Does Mars in a fire sign mean a bad temper?
No. The sign describes the method of applying effort, not the frequency or severity of anger.