https://www.cbr.com/star-wars-han-solo-frozen-carbonite-time/ Before the new canon was established, James Kahn's 1983 novelization of Return of the Jedi stated that Luke and his friends could not rescue Han for about six months. It was explained following his thawing that, "He was, understandably, disoriented, after having been in suspended animation for six of this desert planet’s months—a period that was, to him, timeless." This was considered the official timeframe of his absence, but has since made way for a new explanation since the book is no longer canon.
According to the official timeline of the films, the events in The Empire Strikes Back took place in 3 ABY (After the Battle of Yavin) and the events of Return of the Jedi took place in 4 ABY, meaning that Han was frozen for roughly one year. While most of the planets, like Coruscant, follow a Galactic Standard Year of twelve months, planets like Tatooine have longer days due to their twin suns and measure a year as 304 days instead of the standard 368. This makes it more difficult to pinpoint the exact amount of time that Han was frozen for.
However, a year seems more likely than six months in the new canon, as the recent Star Wars comic series from Marvel has started to cover the events between Empire Strikes Back and Return of the Jedi. So far in the series, the Rebel Alliance has been stretched thin and is trying to rebuild its forces. Meanwhile, Luke is on a quest to complete his training, even obtaining an entirely different lightsaber from his green one. Since the series is still ongoing, there will most likely be even more events to pad the time before the Rebels are reunited with Han, further justifying a year as the correct time frame.