it really depends on how you function. some people can take test till the night before, others (like me) started doing worse and worse on tests even though my hit rate on practice questions were getting better and better.
If your practice test scores are getting lower for no reason, then you should stop taking tests at least 3-4 days before the actual test, and spend the last few days reviewing your error log
. If you're doing better and better with every test, then by all means take your final prep test a couple days in advance and review your errors the last day.