It's like a soap opera, in that there's no really good or bad time to start. Things are just kind of never-ending. But as a general rule, January or April are always pretty good times to get on board, as they start the build toward or wrap up the biggest month of the year.
So, I'm guessing it's sort of like Final Fantasy, where you've got a massive storied history that rarely interacts with prior installments bar the occasional call-back?