The 6 months do not include line training, this period is for the validation of your Japanese ATPL, basic indoc, systems and sim. If you are typed then is only the ATPL ride with the JCAB but if you are non-type rated then is two rides in the sim with the JCAB and one line check if you are a DEC, the entire process including line training is about 8 months, provided you don't require additional sim sessions.
I don't really think is particularly difficult once you understand how to play the game, but it is long, boring and repetitive, probably the most difficult part is to keep motivated for that length of time. One good thing is that the company will put you up in a very comfortable apartment (small but nice) and you can have your family with you, some guys had their espouses here for the duration.