Globally change the number of days a session will be valid in "Account Settings" on the left and then clicking on the "General Settings" tab. Select drop down for "Default Days Session Valid" to edit the number of days you want the link to be active.

You can also change the number of days the link will be valid on an individual test session. The "Session Valid" drop down on the lower right of the screen where you add the applicant information. The default number of days that was set in Account Settings will display. You can choose a different number of days from that drop-down menu, and it also edits the global setting.