A Saturn return is an astrological transit that occurs when the planet Saturn returns to the same place in the sky that it occupied at the moment of a person's birth. While the planet may not reach the exact spot until the person is 29 or 30 years old, the influence of the Saturn return is considered to start in the person's late twenties, notably the age of 27.
An approximate calculation just notes when Saturn returns to the sign it was in at birth. Since it takes about 2 ½ years to move through a sign, it can be said the influence of the return lasts this period.
But by calculating the exact position of Saturn at birth, then comparing the position of Saturn as it moves through the sky, the exact point in time is found when Saturn is very close to the position at birth, and the return is felt at its most intense.
The path Saturn appears to take in the sky is not constant, and in classical antiquity, the 5 planets Mercury, Mars, Venus , Jupiter and Saturn were referred to as “wandering stars”, as their movement appeared to be forward and back against the fixed stars of the sky; for an explanation of this see http://earthsky.org/space/what-is-retrograde-motion
This does mean that depending on where in its wandering path Saturn returns to the exact birth position, there will be a varying length of time that the return will be most strongly felt.
If the movement of Saturn is direct, it can pass over the birth position in a few weeks; if Saturn appears to move back over the birth position, then the effect can last over a much extended period, with peaks as it dances over the birth position.
The two plots show the Saturn return for a birthday 2 months apart. The horizontal line is the birth position of Saturn, the wavy line is the apparent movement of Saturn in the sky.
This Saturn return is exact in the 4th Quarter of 1985
This Saturn return has 3 contacts with the birth extending over most of 1986
Find out when your own Saturn transits are here