The word “dasha” means “state of life” at a particular time. Entering and exiting the dasha most often represents turning points in the life of every person, because they show what kind of karmic influence is catching up with us.
Maha dasha signifies a great period in which a particular planet or graha acts karmically

Surya acts 6 years.
Chandra acts 10 years.
Mangal acts 7 years.
Rahu acts 18 years.
Guru acts 16 years.
Shani acts 19 years.
Budh acts 17 years.
Ketu acts 7 years.
Shukra acts 20 years.

Antardasha is a subperiod within a great period.