Before his operation, Charlie's IQ is around 70. However, following his operation, his IQ begins to rise, as reflected by his word choice, punctuation, and sentence structure.
In the entry for April 16, Burt Seldon tells Charlie that his IQ is around 100, but that soon it should rise to above 150.
On June 6, Alice and Charlie get into a fight. He sadly realizes that it is in part because their levels of intelligence are just as far apart as when the experiment started, but now he is the more intelligent one, with an IQ of 185. This is the highest clearly indicated point of Charlie's IQ, although it's possible that his IQ continued to rise and that he simply did not take the time to measure it.