How much alcohol is a Muslim permitted to drink?

Muslims are not allowed to drink alcohol even in small quantities. The reason behind the prohibition relates mainly to the intoxicating effect that alcohol has on the faculty of reason in human beings. The mind is of utmost importance because it is the recipient of the divine message and legal rulings. In other words, the accountability of human beings is solely determined by their mental capacity. People are not held accountable for anything when they are asleep or under the effect of anesthesia for surgical procedures.
Because drinking was a deeply rooted practice in pre-Islamic Arabia, alcohol was prohibited gradually. This occurred in four stages. The first was the subtle disapproval of alcohol in the Quran when God the Almighty said: “… and grapevines you take intoxicant and good provision” (Quran 16: 67). In this verse, God the Almighty described grapes as wholesome and good but did not describe the state of inebriation they produced as either pleasant or good. This implicit disapproval was well understood by the Arabs who inferred from these words God’s disapproval. The second stage was when God the Almighty prohibited Muslims to approach prayers in a state of intoxication and said: “O you who have believed, do not approach prayer while you are intoxicated until you know what you are saying…” (Quran 4: 43). The third stage was when God the Almighty revealed that though alcohol has benefits, its harm outweighs its benefits: “They ask you about wine and gambling. Say, "In them is great sin and [yet, some] benefit for people. But their sin is greater than their benefit” (2:219). The last stage of prohibition was when God stated clearly its prohibition: “O you who have believed, indeed, intoxicants, gambling, [sacrificing on] stone alters [to other than God], and divining arrows are but defilement from the work of Satan, so avoid it that you may be successful” (Quran 5: 90).
We understand that in a society where pubs are part of the popular culture, Muslims might face some difficulties in changing their lifestyle to refrain from consuming intoxicants. They should therefore try to engage themselves in other social activities which are free from anything prohibited.
And God the Almighty knows best.