Hi charliedan,
Here's some information about timing of sim tests:
By the time you take your sim exams, you should know the material. Ideally, sim exams should be taken after having reaching at least the halfway mark in your GMAT prep schedule. Certainly be sure to take the first one no later than about two weeks before the real test.
Once you begin adding sim exams to your studying regime, you should plan to take one every two to three days. Plan to take five sim exams to thoroughly prepare yourself. These tests should be taken in real time and in test-like conditions. Try not to take too many sim exams—you may burn out.
And remember, no sim tests the day before your actual exam! The day before your exam, do some light studying and try to relax.
Hope this helps!
Regards,
Elizabeth