Shani or Saturn takes 30 years for its one cycle through the horoscope chart. So it takes two and half years in each zodiac house in the horoscope in transit.
Sadhe Saati or seven and half or 'Ezhara Sani' is the total of seven and half years when Saturn transits through zodiac in the three houses :viz. the house where Moon is placed in a horoscope, house just previous to that and the house next to that. The Moon house is one's Raasi and it can be also known knowing the star and time of birth.
It is not correct that Sadhe Saati is always a bad period. For those in whose horoscope Saturn is the benefit giver may get some good results during Sadhe Saati.
For an average person sadhe saati can come twice in lifetime and for many it can come thrice. It is believed in Tamil Nadu that the first sadhe saati in one's life is "Mangu Shani" or when one becomes slow and less famous. If it comes in his education period, he/she may not shine in education, may not win competitions, and mostly slow in activities. It is similarly believed that second round of sadhe saati is "Pongu Shani"- elevating Saturn. This period will give boost to the person. In most people's case it will be around his healthy youth period. So the perso will get rise in salary, positing and family support. So the third is naturally believed as "Marana Shani" or causing death. Naturally the person will be in his advanced ages and will be troubled by diseases and ill health etc.
The remedies for a malefic Saturn is to pray at places where Shaneeswara temples , Navagraha temples. People to wear black or blue colour dress on Saturdays, light sesame lamps(Ellu Kizhi Vilakku) , lamps with gingelly oil. It is also suggested by astrologers to donate black grains (eg Udid, or black gram) on Saturdays. One can recite shaneswara sthothra or sloka and also pray at Lord Ayyappa temples. Feeding crow is also suggested by astrologers.