I started an internship at the beginning of August and at the time, my supervisor and I did not determine and end date. I am not in school right now and the company I'm working for has funding for two full time interns. His thought process was, "we have funds for you for however long you want and since you're not sure when/if you are going to go back to school, let's not set an end date".
That worked perfectly for me, but now, I'm starting to think about the next steps in my life and those may not involve working here. I'm convinced he'd take me as an intern for many months, as they have the funding and the more familiar I get with the project, the more helpful I will be.
However, I would like to pursue other opportunities eventually. I am not ready to have a full time career. I have thought many times about approaching him but I'm not sure how "hey I want this internship to end in December" can come off in a professional and polite way without implying that I don't want to work here and without breaking any bridges or opportunities for future employment. It's like telling your girlfriend you want to break up in a month, but not right now.
I will never again start an internship without an end date, but for now, I need some advice getting out of this situation. It's getting close to when I might want to leave (within 2 months) and I want to make sure they have time to find another intern.