I have now finished the book and was sad that it was done so soon.
I was actually a little disappointed that Sophie changed back into her young self! I’m also not entirely sure how that happened – my understanding is that to some extent the spell had been broken but she kind of kept it going? Anyone else have better insight into that?
I think the Witch of the Waste’s spell didn’t last very long, but Sophie kept it going by reinforcing it: remember, one of the first things she did was look in the mirror and tell herself she’s ninety. I think she finally stopped reinforcing it at the end because she was so busy trying to save Howl, so it faded away.
The witch’s spell broke, but since Sophie saw herself as an old woman with nothing ahead of her for so long, her own magic kept it going.