I did my A320 TR a few month ago. I asked the company if I could get the CBT before hand to study and that turned out to be a very good decision. I had 2 weeks before the 100 question system test. This may sound like enough, but we also had to prepare for MFTD sessions, which ment studying the SOP's as well as knowing your way around the FMGS(FMS in boeings). If you ONLY use the CBT you will NOT know enough for the systems test! You will need to read the FCOM atleast once.
Thats my advice......take care and enjoy.......full flight sim is the most fun
Martin