The Siamese too had been busy planning their defences since Hsinbyushin accession. Their defences had been on alert since they got the Tavoy governor to revolt in mid-1764. King Ekkathat had already launched a massive mobilisation drive across the entire country when Maha Nawrahta came down with his army and retook Tavoy in November 1764. In all, the Siamese southern defences alone comprised over 60,000 troops. Ekkathat placed several of "his best legions" along the entire western arc from the Three Pagodas Pass to Kanchanaburi across Tavoy, to the Gulf of Siam in the extreme south to face off Maha Nawrahta. In the north, the main Siamese defensive line began at Sukhothai and Phitsanulok. The defences farther north immediately facing the northern Burmese army relied on small forts built by local chiefs. Finally, the Siamese relied on a heavily fortified Ayutthaya city, strategically situated at the estuary of two rivers where high rising waters in the rainy season had kept it safe against enemies over the centuries. The Siamese had used their wealth to acquire several thousand muskets and artillery. Some of the cannon were thirty feet  long, and fired 100-lb.  balls.  As was the case in 1760, foreign mercenaries  were hired for Ayutthaya's defence. At least one English ship was enlisted for her broadsides.
Question: How many years passed between the foreign mercenaries being hired for Ayutthaya's defense and the retaking of Tavoy?
Answer: 4