I took my first FL before I even started studying. My thought was "**** it, lets see what I know". Now that my test is way closer (May 19th) I've been pretty nervous going into my most recent FLs.
My test is May 19th and I don't feel nearly "ready". I don't think I'm gonna feel ready. There's just so much to know... but I think you can objectively say that you are "ready" when you consistently see FL scores that would get you into medical school, or are at the average matriculant score of 510. (someone correct me if that number is wrong, I read that somewhere).
Best of luck on your FL. You probably know way more than you think you do (that's been my experience). If you don't get a score you like, remember that it will likely improve with each FL you take, as you build up stamina and testing skills.
EDIT: oh, this post is like a week old