The current window's median is about 6 months.
Theoretically, PCR should reduce the window, however, the actual
performance does not always live to its expectations. The problem may
1. Specimen preparation problems;
2. The virus is not located in circulating periferal lymphocytes at the
early stages of the infection. Therefore, even very efficient
amplification methods are unable to detect.
The are some groups working on other methods for window shrinking with
some impressive results.
You should realize also that the window is an estimate: it is difficult,
if not impossible, to know when the active infection started.