My GMAT students are often surprised when I advise them not to take a practice test.
I don’t advise this for every student on every occasion; there are some legitimate uses for practice tests. In general though, I find that my students take too many practice tests at the expense of other more beneficial forms of study for a given circumstance.
Think of the GMAT like a Mozart sonata. Let’s say you are a pianist, and you want to learn the sonata. Would you begin by playing the whole piece from start to finish? No, instead you would work in small sections. You would identify the sections that are easy, and you would work on those sections just enough to maintain your ability. Mainly, you would be concerned with the difficult sections of the piece, which you would practice slowly and intently. Not until you had mastered those sections would you move on.
After you have put in all that practice time, you want to make sure that you can maintain your ability within the context of the larger piece. That’s when you want to play the whole piece: when you want to check to see whether your prior work is ingrained or whether you forget it when you are distracted by the other demands of the piece.