In this task, you're given a passage, further information available on a particular linked term from the statement, and a question. Your job is to generate the answer to the question by using the information provided. If there is no clear answer obtainable, output 'none'.

Passage: The Battle of Callinicus () was fought in 171 BC between the Kingdom of Macedon and the Roman Republic near a hill called Callinicus, close to the Roman camp at Tripolis Larisaia, five kilometres north of Larissa, the capital of Thessaly. It was fought during the first year of the Third Macedonian War (171-168 BC). The Macedonians were led by their king, Perseus of Macedon, while the Roman force was led by the consul Publius Licinius Crassus. The Macedonians were supported by Cotys IV, the king of the Odrysian kingdom (the largest state in Thrace) and his forces, by Cretan mercenaries and by auxiliaries of mixed nationalities. The Romans had their Italian allies with them and were supported by soldiers provided by Eumenes II of Pergamon, as well as a force of Thessalian cavalry and Greek allies. The battle saw the deployment of troops with cavalry intermixed with light infantry. Although the battle was actually inconclusive because Perseus withdrew before it came to a conclusion, it was considered a Macedonian victory because the Romans suffered heavy casualties.
Link Information: In 179 BC Philip V of Macedon died and Perseus took the throne. Question: How long had he been king when Perseud led the Macedonians in the Third Macedonian War?
Answer: 8