I have to disagree with some of the posts above--I don't think taking two tests in one day is a good idea (I don't even think taking two tests two DAYS in a row is a good idea). The potential burnout factor is really high. Often when people take two tests two days in a row, the second score drops from mental fatigue, and they barely remember taking the second test at all--not to mention that you're not giving yourself proper time to review the exam that you did. *And* you've lost half a day that you could have been plugging content holes, evaluating timing strategies, etc.
Burning through a bunch of exams alone will not help your score nearly as much as methodically reviewing your performance (Generate those assessment reports! Break down past performance! Adjust your study strategy based on that analysis! *Then* and only then, after you've had time to adjust your approach and fill identified content holes-- take another test and see if that made a difference in your score.) In a weird way it feels easier to just do, do, do a bunch of exams, but the more difficult (and ultimately more beneficial) habit is to zoom out and self-evaluate to guide your studies moving forward.
If you feel solid on content and are just brushing up process, a CAT once a week is not a bad idea-- as long as you've got the time to take the test AND do a thorough review. The last week (the week before your test) I'd recommend using a GMAT Prep exam...don't use it before then because you don't get that analyzed report data on GMAT Prep, and you want to save the closest thing to the real deal for *right before* the real deal.
***I tell this story a lot, but I once worked with a young woman who started off at a 480 and ended up in the low 700s. Yes, she studied a lot, but her *system* of study was awe-inspiring. She had a detailed system for reviewing every single question she did, and I could bet money that if she had seen a question *ever* in the past, she would be able to kill that question (and others with similar structures) if she ever came across it again. The moral is: working smarter can yield much better results than blindly working hard.***
I do agree that writing the essay is beneficial for endurance training. If you have limited time, though (and don't we all), don't take the essay on every CAT-- twice should be plenty.