On May 23rd, when Charlie comes into the lab, he records that Algernon bit him. Charlie also notes that Algernon is "unusually disturbed and vicious." By May 25th, Algernon's behavior has gotten worse, and he won't work his shifting lock project. While people tell Charlie he shouldn't assume that Algernon's problems have any implications for his own situation, Charlie strongly suspects that they do by this time.
As a result, Charlie does what he asked not to do: he spends all his time in the lab. By May 31st, he writes that he has to know if and when he will start to regress like Algernon.
On June 4th, Charlie states in a letter that he knows with certainty his own experiment is a failure, meaning he knows that what is happening to Algernon will happen to him. He has done the math, had it checked a dozen times, and can't argue with the facts. He will become mentally handicapped again. Because he is so intelligent at this point, it doesn't take Charlie long between noticing Algernon's strange behavior and connecting it to his own fate.