You set the time length for data retention when setting up your poll. Data may be kept for the length of your license, or for a limited time as it fits your needs.
Log in to your account dashboard.
Click: Create Poll
Choose Number of Days to Retain Responses: 0 (keep responses until you decide to clear it- no time limit) or to any number of days you set. For example, if you wanted to keep the poll responses for 30 day, type '30' in the box.