Everyones probably gonna flame me for saying so, but it's really a personal decision. No one studies like you but you. Only you can decide how much you can retain efficiently in however long you have to prepare.
Personally, I can never retain information conceptually for long periods of time w/out hitting it over and over again. So I'm going to take as long as my school allows (which is June 19th - damn them!).
But, some of my classmates feel that they can cram a ton of info into their heads in a short period of time, and it'll be there just as fresh for exactly 4 weeks - right after which everything floats away. Thus, they're taking exactly 4 weeks for their test (or however long the kaplan program is).
Think about what you've done to help you do well in lecture (study schedule wise)- apply those same principles to board review, and hope for the best. Anyone saying "any longer than 4 weeks is too much", doesnt know you, and is just saying that because he's doing/did that. Likewise, anyone saying "take it as late as possible" doesnt know your mental atrophy rate, and probably did/is doing that too. G'luck.