How an external auditor can judge a company's internal control within a short period of time, like half a day?

Its very difficult to judge the internal control environment of the company in such a short span of time. Assessing the control for external auditor is most important process of understanding the entity and a continuous process during the course of audit. It is also important to assess the internal control to give the audit opinion.

Its better to have a details plan to assess the control environment. Assessing direct and precise controls based on risked identified by the external auditor (based on the understanding and experience of the entity) may help you to close the assessment of internal control within a short period of time. Half a day still looks difficult to achieve.

i would like to get the answer from the person who asks the question to know the answer as he would have encountered such a situation

the appropriate answer is given by the ISA300:

In establishing the overall audit strategy, the auditor shall:

(a) Identify the characteris tics of the engagement that define its scope; (b) Ascertain the reporting objectives of the engagement to plan the timing of the audit and the nature of the communications required; (c) Consider the factors that, in the audito r’s professional judgment, are significant in directing the engagement team’s efforts; (d) Consider the results of preliminary en gagement activities and, where applicable, whether knowledge gained on other engagements performed by the engagement partner for the entity is relevant; and (e) Ascertain the nature, timing and extent of resources necessary to perform the engagement. (Ref: Para. A8–A11)