The Assassin's Creed franchise has seen an annual release every single year since 2009's Assassin's Creed II; if one counted the Nintendo DS game Altaïr's Chronicles, then it's seen a yearly release since the very first Assassin's Creed game in 2007. There have been 17 Assassin's Creed games, spinoffs, and expansions in the past 10 years.
Ubisoft has announced that the next Assassin's Creed game will be launched in 2017. However, the series may no longer be annualised after that, according to Ubisoft CEO Yves Guillemot.
The chief reason is due to the underperformance of the last few Assassin's Creed games. Assassin's Creed IV: Black Flag was considered a major improvement over the disappointing Assassin's Creed III, but it wasn't as big a seller as Ubisoft thought it would be. Assassin's Creed: Unity was a technical disaster, and damaged the waning trust of the fan base, which led to Assassin's Creed: Syndicate being a poor seller despite well above-average reviews.
Guillemot stated that while there is a 2017 Assassin's Creed game coming, there may not be a 2018 game. "The goal is not to automatically come back to an annual cycle, but to come back on a regular basis," he stated. "We can't say every year."
Of course, that may not apply to the spinoffs such as the Assassin's Creed Chronicles series, which are 2D puzzle platformers in the style of Mark of the Ninja. Those titles may have allowed Ubisoft to annualise Assassin's Creed without having to actually touch the main series.