A template's sessions are purged automatically according to the settings under Storage tab > Session Lifetime. However if you would like some templates to keep the sessions for a longer or shorter period than the system wide setting you can change this on each template. To do this edit the template you want to change then click into the Template Info tab. On the left hand side you will a section labeled "Default Session Lifetime". Click on the check box in this section labeled "Override default session lifetime". Set the "Complete session lifetime" and the "Incomplete session lifetime" then click "Save Changes" at the bottom of the page.
Note: increasing the amount of time will increase the amount of storage used. This could result in additional fees to your subscription.