Always write more questions than you need.
It's a fairly straightforward rule -- the stronger students will finish questions quicker than the weaker ones, so as a teacher you are left with a choice between stopping the exercise before all the students have finished, or leaving the faster students hanging around bored after they've finished.
We all typically compromise, leaving the stronger students waiting for a little while, but still not giving the weaker students the chance to finish everything. We formalise this with the immortal line "It doesn't matter if you haven't finished," but in reality, normally it does.
Have a look at most language exercises. You'll typically find that a great many question sets only cover a particular language point or case once, and if you don't answer that question, you don't practise that point. So yes, it does matter if you haven't finished.
The common-sense solution is to write a question set that covers all the points once, then add in additional questions that revisit the same points, but in a more complicated way. Say your minimal coverage of the grammar points can be done in 6 questions -- add another four to make it up to 10. Now you can watch for your weaker students finishing question 6 and declare in all truthfulness that it doesn't matter if they haven't finished... because actually, they have finished -- they just don't know that the remaining questions are primarily time-fillers.
But yes -- primarily time-fillers. As I said, they should also serve to practise more sophisticated uses of the point in question. This way every class is differentiated. All students cover the same basic material, but the advanced students get advanced practise.
Sadly, none of the materials I'm asked to use in class work this way, and I'm forced into the dishonest version of "it doesn't matter if you haven't finished." I would like to see books where the minimal coverage is marked with a line to indicate where "it doesn't matter..." becomes true, beyond which the questions are not strictly required.
This would be helpful. Shame they don't do it.