I've had corns take over 2 weeks to finally shed after I see them in blue. It tends to make me doubt myself when I mark a record as the snake being in blue, and then nothing is found by the next feeding day a week later, and then the following week as well, but then I see one later that day or the next. And it has happened enough times for me to consider it a normal occurrence.
PS - Well, not exactly "normal", but rather "one of those things that are odd but don't be concerned about as it happens and is okay."