It is made up of 'Saadhe-Saat' which means a period of 'Seven & a half' or 7.5 years. A person or his Chandra Rashi or Moon Sign is said to be under the influence of Sadhesati if the Shani or Saturn in Gochar or present tense is transitting through that Chandra Rashi or it's previous rashi or it's next rashi.
When the Shani in gochar (or present) enters the previous zodiac of the Chandra rashi of the person, sadhesati is said to be started on that Chandra rashi (first charan of sadhesati is said to be started). When the Shani enters in the Moon sign of the person, second charan is said to be started & when the Shani enters the next rashi, third charan is said to be started. since Shani takes a period of about 2.5 years in transitting through a zodiac, this way in transitting thorugh three rashis it would take a total of: - 2.5 + 2.5 + 2.5 = 7.5 years.
Three contiguous rashis would always be under the influence of Shani's sadhesati simultaneously. Some rashi would be under first charan, previous to this rashi would be under second charan & previous to this previous rashi would be under third charan of sadhesati. There are a total of 12 rashis. This way 3/12 = 0.25 or 25% rashis are always under the influence os Shani's sadhesati.
But this concept of Sadhesati is not a part of Vedic Astrology & is a part of jyotish of so called astrologers. Mahadasha-Antardasha of grahas plays a more vital role in determining about a person's time phase (good or bad regarding a particular issue of life) at a given time. Sadhesati affects a person more on psychological basis.