It all depends on how well you know the topics already, and how fast you can make your way through the readings, videos (assuming you're using a review course that has those – I think they all do though), and most importantly…the practice questions – all 1,500 of them, or more.
Yes, retention is a problem for some of us not blessed with a perfect memory. That's why you have to always spend time reviewing the old stuff as you study the new stuff. I didn't do enough of that and it kicked me hard in the end. 5-6 months is, I agree too long because you really lose continuity with that long a period. I'd suggest “a few months”. No more than 4 months.
I started studying for FAR in mid-July. In mid-September I got a new job. I was off work for 2 months and used that time to the fullest…I got the bulk of my studying for FAR done during that time (maybe 75% of it). From September to November, I did about 3 hours a night and finished the remaining 25%. By the beginning of December, I was completely burned out. I took it on January 7th and got a 62. If I hadn't burned out, or maybe if I'd taken it RIGHT after I was done studying, I wouldn't have failed. I've moved on to AUD and will re-take later this year.