Thanks to lizbd and (again) Indaloman.
Lucy Thorpe could have been held in prison until, say, 1845; but your researches, lizbd, appear to have revealed no deaths of Lucy Thorpes between 1840 and 1848. I don't think that she'd have been transferred elsewehere - this didn't seem to happen in Norfolk as far as I can make out.
But, given the crime of infanticide, it is possible that Lucy was respited (reasons of mental unbalance, perhaps) and committed to a term of sentence. That's the only explanation I have at the moment. Hence the usefulness of newspaper accounts. Should anyone, therefore, be pursuing news through Norfolk Chronicle and Norwich Mercury for 1844-6 and finds a mention, please let me know.
Meanwhile...
roly