I heard that Lord Vishnu will take birth as Kalki avatar when the adharma will be on the apex. So what is the apex of adharma?
Yes, Lord Vishnu will take the avatar of Kalki at the end of Kali yuga when the adharma increase to an intolerable measures. The conditions are said on second chapter of twelfth skandha of Śrīmad-Bhāgavatam. I am posting some of the characteristics from the chapter.
Wealth alone will be considered most important characteristic of a person's conduct.
In Kali-yuga, wealth alone will be considered the sign of a man’s good birth, proper behavior and fine qualities. And law and justice will be applied only on the basis of one’s power. [12.2.2]
The spirituality of a person will diminish. Rituals will be performed only for the name sake and for reputation.
A person’s spiritual position will be ascertained merely according to external symbols, and on that same basis people will change from one spiritual order to the next. A person’s propriety will be seriously questioned if he does not earn a good living. And one who is very clever at juggling words will be considered a learned scholar. [SB 12.2.4]
Purity of the people (both physically and mentally) is lost.
A person will be judged unholy if he does not have money, and hypocrisy will be accepted as virtue. Marriage will be arranged simply by verbal agreement, and a person will think he is fit to appear in public if he has merely taken a bath. [SB 12.2.5]
Earth's population will be increased very much and most of them will be unjust and corrupt people. People with political power irrespective of their social class rule the people.
As the earth thus becomes crowded with a corrupt population, whoever among any of the social classes shows himself to be the strongest will gain political power. [SB 12.2.7]
The rulers will be no different from thieves and highway bundits. Rulers will collect excessive taxes from their people. There will be many famines. Unity between the citizens will also disappear as they quarrel among themselves.
Losing their wives and properties to such avaricious and merciless rulers, who will behave no better than ordinary thieves, the citizens will flee to the mountains and forests. Harassed by famine and excessive taxes, people will resort to eating leaves, roots, flesh, wild honey, fruits, flowers and seeds. Struck by drought, they will become completely ruined. They will be further tormented by quarrels, hunger, thirst, disease and severe anxiety. [SB 12.2.8-10]
The maximum life span of humans will be fifty years.
The maximum duration of life for human beings in Kali-yuga will become fifty years. [SB 12.2.11]
At the end of the Kali yuga, heinous crimes increase. Physical changes also occur and the creatures size will be greatly decreased. Religions will become atheistic.
By the time the Age of Kali ends, the bodies of all creatures will be greatly reduced in size, and the religious principles of followers of varṇāśrama will be ruined. The path of the Vedas will be completely forgotten in human society, and so-called religion will be mostly atheistic. The kings will mostly be thieves, the occupations of men will be stealing, lying and needless violence, and all the social classes will be reduced to the lowest level of śūdras. Cows will be like goats, spiritual hermitages will be no different from mundane houses, and family ties will extend no further than the immediate bonds of marriage. Most plants and herbs will be tiny, and all trees will appear like dwarf śamī trees. Clouds will be full of lightning, homes will be devoid of piety, and all human beings will have become like asses. At that time, the Supreme Personality of Godhead will appear on the earth. Acting with the power of pure spiritual goodness, He will rescue eternal religion. [SB 12.2.12-16]
These are the conditions when Lord Viṣṇu takes the incarnation of Kalki and reestablishes eternal dharma. That will mark the beginning of the Satya Yuga. The incarnation marks the end of the Kali yuga and the beginning of Satya yuga.