Practice exams are the most informative thing you will do during preparation, and the easiest to turn into an expensive ritual. The difference is almost entirely in the spacing and the review.
Take the first one early
Early enough that the result is still useful. An assessment taken when you feel ready confirms what you already suspected; one taken while you still feel unprepared tells you where to spend the weeks you have left.
That first result is often uncomfortable. It is supposed to be. Its value is proportional to how much time remains to act on it, which means the discomfort is the point rather than a reason to postpone.
Students who delay their first assessment until they feel ready almost always discover their weak areas too late to do much about them.
Space them out
Every ten to fourteen days is a reasonable rhythm. That is long enough for studying between assessments to actually change something, and short enough to catch a plan that is not working.
The common mistake is clustering assessments in the final two weeks. At that point they cannot change your preparation. There is no time left to redirect, so they stop being diagnostics and become anxiety generators with a score attached.
Sit them properly
Full length, timed, no pausing, at roughly the time of day your real exam is scheduled. Stamina is a genuine variable and it is the one thing a shortened or interrupted practice cannot measure.
If you have only ever taken assessments in two halves with a long gap, you do not yet know how you perform in the last block of a long day, which is exactly where results are lost.
Budget the review
An assessment costs the exam itself plus a review that takes about as long again. If you cannot afford both, take fewer.
The review is where all the value is. The score is one number with wide error bars; the question-by-question walk-through tells you which errors were content gaps, which were failures to recognise a presentation, and which were pacing or misreading. Those three categories point at three completely different fixes.
Your question bank should help with that sorting rather than leaving it to memory. QBankly breaks results down by system and discipline, which makes the pattern across several assessments visible.
How many in total
For a typical dedicated period, three to five, spaced as above, with the last one far enough out that you can still act on it, not the day before.
More than that and you are spending your remaining time measuring instead of improving. Fewer and you are steering with very little information.
Reading the trend
One result is a measurement. Three results are a trend, and the trend is what you should actually be reading. A flat trend across several weeks means your study method is not working and needs changing, which is far more useful to know than any individual number.
What to do with a result that surprises you
A result well below your trend is usually conditions rather than knowledge, poor sleep, an unfamiliar time of day, a form weighted toward something you have not covered. Check those before you rebuild your plan around one bad morning.
A result well above your trend deserves the same scepticism. One good day is not evidence that you can stop, and the students who ease off after a strong assessment frequently find the next one has drifted back.
The rule in both directions: a single point does not overturn a trend. Wait for the next one before you change anything substantial.
Assessments during the final fortnight
By this stage, an assessment can no longer change your preparation. There is not enough time left for a redirected week to show an effect.
That does not make one worthless, a final check confirms your pacing and gives you one more full-length rehearsal, but its purpose has shifted from diagnosis to familiarisation. Treat it accordingly, and do not let a mediocre result at that stage trigger a panic rewrite of a plan that is nearly finished.
Leave the last few days clear. An assessment two days before the exam buys you almost nothing and costs you rest at the point where rest has the highest return.
If you change one thing
Take the first assessment earlier than is comfortable. Its value is proportional to the time left to act on it, and that time only ever shrinks.
Frequently asked questions
Is there such a thing as too many practice exams?
Yes. Each one costs most of a day including review, and beyond a certain frequency you are measuring rather than studying. If assessments are crowding out question practice, you are taking too many.
Should I take one under full timed conditions?
At least the later ones, yes. Stamina across a full-length exam is a real variable, and it is the one thing shortened practice cannot tell you about.