I didn't realise this was completely normal until a few months ago when I saw a Remedial massage therapist who specifically deals with the breasts and lactating women (current and post lactating) and he told me that the breasts don't just empty by themselves it can take quite a long time for some women. Other women can dry up quite fast. Through remedial massage he helps to drain the breasts through lymphatic drainage techniques etc.
very interesting stuff. He told me a story about a woman who hadn't BF for 7yrs and he expressed (after massage) stale milk from her! incredible it can stay in there for so long huh?