This is kind of a boring answer, but it really depends. From Appian's account, for example, the sack of carthage was nearly a week of brutal street to street fighting and the city was more or less destroyed in the course of this, as houses and walls were turned into fortifications and counter fortifications. With the sack of Corinth, however, according to Dio the city was essentially abandoned by the time Mummius got there, and what followed was a well organized looting. Livy records that the sack of Veii was because Camillus essentially lost control of his soldiers, and while the story itself is so old that it is probably not specifically creditable, it at least shows that this was plausible. And Alexander's sack of Persepolis was a planned orgy of destruction. This was how the sack of Athens began, but Sulla had a change of heart and decided to end it, which shows that a strong willed commander could maintain control of his army.

So really a "sack" isn't a single event, and how it occurs depends greatly on the character of the general and army and the events leading up to it.