You have a good question but I think not even Home Office answered that unless I missed it buried under lots of text.
Best practice would be to take the test when you have met all other conditions and ready to apply for Citizenship.
Who knows this test might not be a requirement year from now and you wasted your time and money. If you read on this Web site, you will be ready in a day or two working at least 6 hours a day. Beauty is that you don't even have to buy a book as all material is provided here.
Hope this helps
