*I just had this thought--If you are reading this in high school, you may be reading an abridged version of the story from your literature book-- in which case, you may not have the scene with Dr. Nemur's presentation from "Progress Report 13." Check and see if you have the full version of Keye's story or not--because it will make a difference for your answer.
Charlie, and the reader as well, both begin to have an inkling that his intelligence may not be permanent as he listens to Dr. Nemur's presentation in "Progress Report 13." Charlie even realizes that Nemur did not take into account his rapid rise in intelligence, and that now, Charlie may even regress into a lower IQ than before the experiment. Charlie becomes so upset that he releases Algernon from his cage, and then takes the mouse with him to New York.
Keyes also reinforces his foreshadowing as Charlie begins to take note of Algernon's agressive tendencies. Algernon's reversion is a huge warning sign that the same fate may be in store for Charlie.
You realize that Charlie's intelligence isn't permanent when Algernon (the rat) starts to forget the maze. Before the rat was able to memorize the maze and escape the maze quickly. This speed represented how smart the rat was. Charlie and Algernon were experiments that had relatively the same reactions and results. With the surgery, both Algernon and Charlie both became super smart. With Alergon's memory and knowledge decreasing, it can be predicted that the same will happen with Charlie's intelligence, and might even be at a lower IQ than he was before the surgery.
Charlie realizes the intelligence is not pernament when the mouse starts to forget the maze. The mouse sucess starts the decreases, he realizes, that after his peak, his intelligence will regress as fast as it was increasing.
when the mouse cant remember the maze.