I know some saints perform the Chatur Masya Vrata. I want to know for what reason. Also what is the duration of this vrata?
The term Chaturmasya derived from two words Chatur - meaning four and Masya meaning Month, collectively called four-months. The Chaturmasya Vrata is a Vrata to be observed for a period of four months.
According to the Vedic authoritative source 'पक्षा वै मासाः' one ‘Paksha’ or a fortnight is taken as one month, and traditionally the Vrata is observed only for two months.
The Chaturmasya period begins in the month of Asadha (June-July) from the day of Ekadashi called Sayana-Ekadashi, in the fortnight of the waxing moon. The period ends in the month of Kartika (October-November) on the Ekadashi day known as Utthana-ekadashi, in the fortnight of the waxing moon.
During the rainy season, Sanyasins observe Chaturmasya and stay at one place to meditate on the Absolute Reality and instruct their disciples in the Spiritual lore.
Is this Vrata for Saints ? NO
This Vrata is not considered as difficult, The Vrata can be observed by all sections of the population. It does not matter whether one is in Grhastha Ashramas or a Sanyasi Ashramas, its compulsory for all Ashramas.
The Chaturmasya Vrata requires different vows to be observed, the real purpose behind the vow taken during these four months is to minimize the quantity of sense of satisfaction from desire, vows enlisted in this LINK.