Teacher: In this task, you're given passages that contain mentions of a time duration related quesry and we are supposed to write answer to a question that involves event “frequency", which refers to how often an event is likely to be repeated. For example, "taking showers" typically occurs ~5 times a week, "going to saturday market" usually happens every few weeks/months, etc. 
 Note that a lot of the questions could have more than one correct answers. We only need a single most-likely answer. Please try to keep your "answer" as simple as possible. Concise and simple "answer" is preferred over those complex and verbose ones.  
Teacher: Now, understand the problem? If you are still confused, see the following example:
Sentence: Namibia conducts a census every ten years. After independence the first Population and Housing Census was carried out in 1991, further rounds followed in 2001 and 2011. The data collection method is to count every person resident in Namibia on the census reference night, wherever they happen to be. This is called the de facto method. For enumeration purposes the country is demarcated into 4,042 enumeration areas. These areas do not overlap with constituency boundaries to get reliable data for election purposes as well. 
Question: How often does Namibia conduct a census?
Solution: every ten years
Reason: Census is generally conducted every ten years in most countries like Namibia.

Now, solve this instance: Sentence: Seattle annually sends a team of spoken word slammers to the National Poetry Slam and considers itself home to such performance poets as Buddy Wakefield, two-time Individual World Poetry Slam Champ; Anis Mojgani, two-time National Poetry Slam Champ; and Danny Sherrard, 2007 National Poetry Slam Champ and 2008 Individual World Poetry Slam Champ. Seattle also hosted the 2001 national Poetry Slam Tournament. The Seattle Poetry Festival is a biennial poetry festival that (launched first as the Poetry Circus in 1997) has featured local, regional, national, and international names in poetry. 
Question: How often is the Seattle Poetry festival held?
Student:
biennial