The total time span of these planets add upto 120 years like every planet has its own great period e.g sun is having a Mahadasha of 6 yrs and Venus is having a mahadasha of 20 yrs. It is unlikely that any person will repeat these mahadashas.
The period of particular mahadasha will start from the time of birth and is based ont e planet ruling the birth nakshatra/constellation.
These periods depict specific transitions and transformatiion in our life. Mahadasha influence different aspects of our life ruled by that planet. The effect will be for good or bad is based on the placement of the planet in an indiviual’s horoscope.
Mahadasha is divided into nine segment called antardashas. it is also known as sub ruling periods. which runs in the same order but the time span is not same. Antardasha require quite bit of work in order to see what will happen in that peiod