Reserve a whole morning/afternoon for it. The test and the interview themselves are quick, but there's a lot of waiting - you have to wait until everybody is in the room, the agents will explain the test, you'll have to wait to be called for interview after the test and, if your family is in the same process, you'll have to wait for each member to be interviewed. I could enter the room at 13h, left after 16h. Most of the time was waiting between test and interview - I couldn't see a particular order, they don't necessarily call first who finished the test first.
As for the test center, the letter will indicate where you have to go.