It depends on the type of math being tested and the students who are being tested.
Firstly, ALL exams are timed. However, some exams are designed to be easily finished in the time allowed.
The concept of a timed exam is that it would be impossible or extremely challenging to complete the exam in the time allowed.
The purpose of this should be to measure a persons skill or ability, not to measure their knowledge.
Since most elementary schools measure knowledge, not skills or ability, then a timed exam would not be appropriate for them. Also, if the test is only a knowledge test, then again a timed exam is not appropriate.
A timed exam is appropriate when you are measuring a person's mastery of skills and their ability to apply knowledge to complex problems.
After rereading your question, I think the true question is at the end - the true question is "Should I drop topics to allow students time to finish." In fact, whether you drop topics or not, the students will not "finish" the exam, either due to lack of time or lack of questions!
I think the best solution is to create two exams if one exam cannot adequately cover all the topics, or arrange for a special double-long testing session.
Further, the distribution of your topics should be reconsidered if a certain period's teaching cannot be tested in one hour. If it cannot be tested in the allotted time, it probably cannot be learned in the alloted time either. If that is the case, then the student is doomed regardless of the testing format.