Hi Chih, not sure if there's a time limit on taking the essay, but I STRONGLY encourage you to take it as soon as possible. I waited about a week and a half after submitting my assignment to take the essay. I figured all the details of my project would still be fresh in my mind, but I found it hard to recollect certain decisions I had made. The sooner the better is all I have to say. And no, you don't have to wait for a confirmation of receipt from Sun before taking the essay.