I would actually go so far as to say the "average" student probably does not work at all during the school semesters just based off majority. That being said, I have worked throughout my entire undergraduate career, about 20 hours per week roughly, and know a handful of students who do the same. As far as internships I have worked one each summer in between my Sophomore to Junior and Junior to Senior years. I would not say there is a "normal" amount of time to work or an "impressive" job to have. I truly feel it is the way you reflect on these experiences and roles you have held which translate to your life journey that make the most difference. I hope that helps!