answer from http://dnnblog.venexus.com/DotNetNuke+4+Persistent+Login+Remember+Me+For+More+Than+1+Hour.aspx

The "Remember Me" functionality during DotNetNuke 4 login works for only 1 hour by default. Previously, this had been set to 50 years. In order to change persistent login to an extended time, a change to the web.config file is necessary. In the web.config you should see this:

<authentication mode="Forms">
      <forms name=".DOTNETNUKE" protection="All" timeout="60"/>
</authentication>

To increase the period of persistent login to 7 days would be this:

<authentication mode="Forms">
      <forms name=".DOTNETNUKE" protection="All" timeout="10080"/>
</authentication>

To increase the period of persistent login to 1 Year would be this:

<authentication mode="Forms">
      <forms name=".DOTNETNUKE" protection="All" timeout="525600"/>
</authentication>

Hope that helps!