I don’t think there will ever be a total replacement. However, for me it took almost no time at all for my kindle to replace traditional hard copy. The reasons:
1) I don’t have to decide which book to take along in the morning as I have my entire library plus access to a book store instantly available at all times.
2) The form factor is small enough that it slips easily into a jacket pocket or my backpack so I always have it with me, thus giving me more opportunities to read for pleasure during those unexpected wait times.
3) It lays flat so it is darned easy to prop it up on the console of the cardio equipment at the gym.
4) It uses 100% recycled electrons rather than paper pulp.