In a novel, of course, the writer can make the characters use them and use them up at the appropriate time (provided the hand of the writer is sufficiently disguised). But they seldom do. Even things where they deliberately wait until the appropriate time -- the horn of Queen Susan in Prince Caspian -- tend not to be used up.
The only non-trivial thing I can think of in any novel is Lucy's cordial, and I note that she was chary of its use. I don't think it got used up entirely before Narnia ended.