Famous professors' email inboxes are constantly overflowing with papers they genuinely want to read, recommendation letters they need to write, and administrative tasks that they have to take care of. I would say the meaning of "a few days" is "as soon as I can".
Since you have e-mailed him twice already, I would not do so yet again. Unless you submitted the paper for publication at a journal this professor edits, he is under no obligation to respond.
You said that you asked him "if the proposition there makes sense". I warn you that this is a bit awkward. It is perfectly fine (and indeed I highly recommend it) to send your work to scientists whom you think may be interested, and to ask for comments. But in most circumstances, it's a bit impolite to ask scientists whom you don't know to check your work.
All that said, I think it's more likely that you'll get a favorable response than not. It is December 11 today, and at most US universities final exams are concluding right about now -- which keeps most professors very busy with helping students and grading exams. Very possibly the professor in question has postponed everything which is (to him) not urgent, but fully intends to reply once exam season is over.