Many faculty ask students for feedback around the middle of the semester in order to be able to make mid-course adjustments. And then, of course, there is the end-of-semester Student Ratings of Instruction instrument that is intended both to furnish numerical data for evaluation and also to elicit student comments helpful for improvement. The problem with relying on the Student Ratings of Instruction, though, is that the results are returned to us too late often to do anything about whatever points the students raise.
There are other (probably better) ways to get end-of-semester student feedback for your own purposes of improvement, and I'll be writing about those in the upcoming November Newsletter from the Center for Faculty Development.
In the meantime, Susan Codone offers this simple yet effective way to elicit student feedback at any time in the semester: The Plus/Delta method. Why wait for the middle or the end of the semester when sometimes we want to know what's working for students and what's not working for students at multiple points in the term?