The length of the window period really depends on what type of HIV test is being used.
The HIV tests we use in BC Canada are similar to other HIV tests used around the world and can look for Antibodies (what your body makes in response to HIV being in your body), Antigens (proteins from the HIV virus) or NAAT (Nucleic Acid Amplification Test, which looks for the genetic material of HIV in the blood).
Some of the HIV test you can get use a combination of these, for example the 4th generation HIV test we use in BC Canada looks for antibodies and antigens. With this test we find that it is 95% accurate by 6 weeks and would give you a final result at 3 months.
The 3rd generation HIV test we used to use, just looks for antibodies and had a window period of 3 months as well.
It is not so much that countries have different window periods, but more that different places will use different tests. To find out the correct window period it is always best to ask the health professional what HIV test they are using and what is the window period for that test.
Another problem can also be found with the information provided on the internet. I have found that there is a lot of old information on the internet that has not be removed or changed. I remember information about window periods being 1 year long but that was 20 years ago.
Let us know if you have any further questions or concerns.