As a CFI, I can tell you that the length of the FR pretty much depends upon the student's level of preparedness. I have done flight reviews for pilots that were pretty much pro forma since they were very active and "current" on recent changes and it was easily demonstrated in an hour of conversation, spread out between pre-brief and debrief. Other pilots haven't flown much in the last two years (or decades) and need much, much more. Somehow, most of them still understand the "1 hour" requirement as the standard time a flight review should consist of instead of understanding that it is a "minimum". I can tell in the first five minutes how familiar a pilot is with the rules and regs. I had one gentleman who couldn't name the required documents for legal flight, asking him to describe airspace weather requirements was taxing... and we're in very close proximity to B airspace. Oftentimes, a single hour simply isn't enough to get a pilot caught up on things they may have forgotten or are unfamiliar with due to lack of use, even in the course of a year or two. This is where taking a prep course CAN make your FR more efficient.
Talk to the CFI you're using. To me, someone taking the initiative to use a FR prep course is proactive and interested in the value of the FR. You may even find that it leads you to bring questions for your CFI, and make you a less passive participant.