Sita stayed in Lanka for about an year (or slightly less). Let's see the time line. Ravana kidnapped Sita, Ram started looking. After few days he found Hanuman and Sugreev. I would say that took around a month. Then killing bali and Sugreev coronation… another month perhaps. Then 4 months of rainy season. read more
Few legends infere that Sita stayed in Lanka under the captive of Ravana for 12 Months, However Padma Purana states and confirms that Sita Lived in Ravana’s House for the brief period of 11 Months and 14 days to be precise. read more
And just around 2 months later from the day of Hanuman's meeting with maa Sita (3rd November 7292 B.C), the final war, to rescue maa Sita, had started. So Ravana kept maa Sita confined for around 12 months or roughly 365 days. read more