The reason it will take a long time is it seems the textbook producers are taking the same low road that magazine producers are taking, charging a higher cost than just buying the print version.
Take coursesmart for example, they offer a "subscription" service where you can download an ebook for 180 or 360 days, depending on the course length. After that period of time you have to "resubscribe" or you cannot access the material any longer. There is a decent discount from the print version doing this, but I'd rather have the book forever as if I bought the print version. It is a cheaper alternative for the student though, but I'll bet that it's still more expensive than getting a used copy of the printed book.
Still, you can't argue with the incredible convenience of having all your books in a little 2lb. package to carry around with you, annotate, bookmark, etc. Just don't expect it to be cheap.