Task: Question Generation
Input (English): Passage: The Confederation-controlled Ava was next. Bayinnaung made extensive preparations, assembling a large invasion force . In December 1554, the Toungoo land and naval forces invaded. Within a month, the invasion forces held complete command of the river and the country. On 22 January 1555, the southern forces took the city. Bayinnaung then planned to follow up to the Confederation's home states to prevent future Shan raids into Upper Burma. With the entire Irrawaddy valley under his control, he was able to raise a large army of 24,000 men, 1200 horses and 60 elephants, and invaded the Shan states in January 1557. The massive show of force worked. Shan states one after another submitted with minimal resistance. By March 1557, Bayinnaung in one stroke controlled all cis-Salween  Shan states from the Patkai range at the Assamese border in the northwest to Mohnyin , Mogaung  in present-day Kachin State to Momeik  Thibaw , and Mone  in the east.
Output (English): How many years after southern forces took the city of Ava did the invasion of the Shan states to control more land occur?