I took 3 TPR practice full lengths, the first three I did, and they were much harder than the real thing.
Once the e-mcat.com tests by the AAMC became available, i ate those up. I avoided the older ones, and did tests 6, 7, 8, 9, and 10 from that set.
No more than one a week, and I spent MUCH more time reviewing the tests I took than I ever did taking tests. The stuff you learn AFTER the exam far outweighs the actual testing experience.
Also, I highly recommend you do the exams in one sitting, all the way through. I usually skipped the writing section, which most people don't fret too much about. It's important to simulate the testing experience under harsh timed settings. It also builds up brain stamina, cause you will be worn out by the time you hit biological.