From the Guidelines

Taking 1500 mg of Tylenol (acetaminophen) in a single dose instead of the intended 1000 mg is considered a moderate overdose.

The standard adult dose is 1000 mg every 6 hours as needed, and a one-time dose of 1500 mg is not expected to cause severe liver damage, but it is essential to monitor for signs of toxicity, such as nausea, vomiting, and abdominal pain, for at least 24 hours 1.
If symptoms persist or worsen, medical attention should be sought promptly, and treatment with N-acetylcysteine (NAC) may be initiated if necessary, typically within 8-10 hours of ingestion 1.

NAC is an effective antidote for acetaminophen poisoning and can be given orally or intravenously, with few side effects 1.
The standard acetaminophen toxicity nomogram may aid in determining the likelihood of serious liver damage, but cannot be used to exclude possible toxicity due to multiple doses over time, or altered metabolism in the alcoholic or fasting patient 1.
Administration of NAC is recommended in any case of acute liver failure in which acetaminophen overdose is a suspected or possible cause, and should be given as early as possible, but may still be of value 48 hours or more after ingestion 1.

From the Research

Taking 1500 mg of Tylenol (acetaminophen) in a single dose instead of the intended 1000 mg may lead to an increased risk of acetaminophen poisoning 2.
According to the study, patients 6 years of age or older should be referred to an emergency department if they have ingested at least 10 g or 200 mg/kg (whichever is lower) of acetaminophen 2.
The toxic dose of acetaminophen may vary as a function of individual glutathione levels, but toxicity is likely to occur after a minimum ingestion of 140 mg/kg 3.
Treatment with N-acetylcysteine (NAC) within 10 to 12 hours following ingestion can reduce mortality to zero and diminish liver toxicity 3.
The use of a single 4-hour acetaminophen determination may lead to failure to recognize patients with potentially toxic Tylenol Extended Relief (TER) ingestion, and it is recommended to obtain at least one additional acetaminophen determination at least 4 to 6 hours after the first 4.
NAC treatment should be initiated if either level is above the nomogram line, but not if both levels fall below the nomogram line 4.
Efficacy of oral N-acetylcysteine decreases with further delay, and treatment is still indicated at least as late as 24 hours after ingestion 5.

Hepatotoxicity developed in 6.1 percent of patients at probable risk when N-acetylcysteine was started within 10 hours of acetaminophen ingestion, and in 26.4 percent of such patients when therapy was begun 10 to 24 hours after ingestion 5.
Among patients at high risk who were treated 16 to 24 hours after an acetaminophen overdose, hepatotoxicity developed in 41 percent 5.

Activated charcoal can be considered if local poison center policies support its prehospital use, a toxic dose of acetaminophen has been taken, and fewer than 2 hours have elapsed since the ingestion 2.
Gastrointestinal decontamination could be particularly important if acetylcysteine cannot be administered within 8 hours of ingestion 2.
Hemodialysis may be used as adjunctive therapy to enhance the elimination of acetaminophen in severely poisoned patients who cannot tolerate NAC therapy 6.