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.  
One 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 is here: every ten years
Explanation: Census is generally conducted every ten years in most countries like Namibia.

Now, solve this: Sentence: The government of the Marshall Islands operates under a mixed parliamentary-presidential system as set forth in its Constitution. Elections are held every four years in universal suffrage (for all citizens above 18), with each of the twenty-four constituencies (see below) electing one or more representatives (senators) to the lower house of RMI's unicameral legislature, the Nitijela. (Majuro, the capital atoll, elects five senators.) The President, who is head of state as well as head of government, is elected by the 33 senators of the Nitijela. Four of the five Marshallese presidents who have been elected since the Constitution was adopted in 1979 have been traditional paramount chiefs. 
Question: How often are elections held in the Marshall Islands?
Solution:
every four years