[Zope] Persistency of Variables Placed in the SESSION attribute of the REQUEST object

Hello. Does anyone know if there is a limit on how long a varible placed in the SESSION attribute of the REQUEST object can persist? I am setting a session variable using the following: <dtml-call "REQUEST.SESSION.set('newvar', REQUEST.form['chosenYear'])"> The variable chosenYear is submitted via a form and this value is placed in newvar. The value of newvar ceases to exist after I let my browser session lie idle for a while. Any help would be greatly appreciated. Thanks. - Asad