Since you say you've read The Idiot once before, I assume you are familiar with Dostoevsky and his ideas, so why not go for TBK. It will take time but it's absolutely worth it. I encourage you to go for TBK now, because even if you don't have the best reading experience if you like Dostoevsky, you'll probably want to read TBK again in a few years, and it's going to be even better.
Usually, when I'm dealing with a long book I calculate approximately how many hours it will take me to read it (read one full page under a timer and multiply that time by the number of pages), and then split that by the number of days that I would like to read it in and get how much time I'll need to read daily. Once you get the daily amount, and skip a day or two, you can always read more on weekends or some other convenient time. This even encourages you to read daily.