Presently, using the most contemporary tests, the average time is now 25 days. The time it takes for tests to show positive depends on two factors. One is the sensitivity of the test itself. The other is the time that a persons body will begin producing antibodies for the test to find. There is no indication that the mode of transmission, nor any other specific factor, affects the time it takes for an individual to start producing antibodies.
Different people respond to infections differently.
Some people naturally take longer than others to produce antibodies. There have been people who have taken longer than 6 months to show positive, but these cases are extremely rare. Like any other disease and test in medicine, there will always be rare exceptions and outliers, as far as HIV testing is concerned. However, the chances that an infected person will not show positive at 6 months are extremely remote. Therefore, testing beyond 6 months is not necessary in the vast majority of people.