Hi Linsey,
Good question because of just what you said about not wanting to wear out your students with your feedback requests.
I ask for feedback after the completion of a major project, activity or group exercise. I make it very simple and quick. I give each student a 3X5 card and ask them to make any comments about the just completed effort. I don't ask "what you liked or didn't like" because I don't want to set any parameters to their responses. They are very candid and I have not had any student over the years vent. They are constructive even if they didn't do as well as they expected or didn't like what we did. This feedback method takes 3-4 minutes at the end of the class so the students are losing any instruction time.
I have a more formal feedback form I use at the end of the course for my own growth for the next class. This is different from the formal course evaluation my college uses. This evaluation is worthless because it asks such generic questions because it is used for every course in the college.
Gary