DON’T take a practice CAT within five days of the real test
Would you run a practice marathon a few days before a real marathon? Of course not! You risk tiring yourself out or (mentally) injuring yourself (by reducing your confidence) just before the real test.
If your score is not where you want it to be, postpone the test; you are not going to change it substantially by taking a practice CAT at the last minute (or doing anything else).
DON’T go months without taking a CAT
When someone does this, the impetus is usually anxiety. You feel nervous that you will not get the results that you want, so you avoid getting any results at all. Alternatively, maybe you plan to study everything and then when you take the test, you are confident that you will get the score you want… but practicing without any CAT data is going to cause you to build bad habits (such as spending too much time on a question) and fail to build good ones (such as learning how and when to cut yourself off and guess).
If your last CAT was so long ago that you are no longer sure what your strengths and weaknesses are under testing conditions, it is time to take another CAT.
In short, do take a CAT pretty early on in your study process. Then analyze the results and use that analysis to inform your study plan. When you have addressed a substantial proportion of the major issues identified via that analysis, take another CAT. Most of the time, you should be able to find at least two to three weeks’ worth of issues to address after every CAT.
Once you have your score where you want it to be, start your final review. During this phase (which typically lasts a couple of weeks), plan to take one CAT two weeks before and another CAT one week before your real test date. Read the article “The Last 14 Days: Building Your Game Plan” to learn what to do with this data.
Good luck and happy studying!