User logged out automatically - too early

Hi. Some users have complained that they have been automatically logged out when they try and save their work (system allows users to enter paragraphs of information as data.)
They write their data into the form, go off and have a cup of tea or something, come back and click my 'save/update' button, and are automatically logged out. Here's some info about my app:
asp.net 2.0, sql server 2000, standard sql membership provider, 
 Little bit of my config:

<authentication mode="Forms">
            <forms cookieless="AutoDetect"/>
        </authentication>
        <membership
        defaultProvider="amsMembershipProvider"
        userIsOnlineTimeWindow="90">
   ...