What can a professor do about an exam that was too long for the allotted time, after the students have completed it?
This is an excellent example of when a professor may choose to grade on a curve.  Grading on a curve normalizes the exam against observed performance rather than an absolute standard, and is a good way to re-normalizing when an exam proves unexpectedly challenging.

Your professor, however, might not choose to do so, or might choose to set the curve in a way that doesn't help you as much as you might like.  The key is what's happening in the class relative to the actual educational goals of the professor.


If the exam was harder or longer than intended, and the professor believes that the lower grades reflect that, rather than the knowledge of the students, then they are likely to grade on a curve.
If, on the other hand, the students are showing evidence that they do not understand things as deeply as they are required to, then the professor may not curve or may adjust with a curve less than you hope.  


It's really impossible to know which case without knowing the details of the exam and peoples' performance.  A "long but easy" test sometimes is testing for deep facility that allows people to be extremely fast.  For example, think of the arithmetic exercises sometimes done in elementary school, which can only be solved quickly enough if the student has effectively memorized and internalized arithmetic, rather than working out answers from first principles (e.g., counting on fingers).  The same principle can sometimes apply in undergraduate and graduate education as well.

Bottom line: maybe the professor will choose a generous curve, but there are reasons that may argue against it as well, depending on the circumstances.