Do whatever is right for you. My invite came right before last Christmas, and I was on the downside of one of the worst illnesses I'd ever had. The holiday plus being sick meant that I waited almost the full week before taking it. I took it on a Friday, which was foolish -- don't do that unless you're prepared to wait the weekend to hear!
I heard the good news on the next Tuesday.
I also logged in, logged out a few times. No issues there. Ideal to do it all in one sitting, but just saying it's not impossible. I don't think they care when or how you do it, as long as you follow the directions that UA sends you.
I've heard of people blowing through it in 20 minutes, passing, and also guys who didn't read the questions well and failing. All told, it took me an hour and a half. I wanted to be careful about the always/sometimes/nevers.
One last bit, I'd say out of the 400 questions, 380 to 390 of them will be easy. It's the 10-20 that hurt your brain. Be positive, and good luck!
C72