In order to modify this, you must be a Platform Manager.  The following steps will lead to the Timeout settings.
  1. Navigate to Core > Security
  2. Select Authentication settings
  3. Under the Authentication Rules page enter the number, in minutes, into the Session Timeout box before the password is required
Note: This will affect all users.  The default value is 20 minutes.  Users that have access to Credit Card or SSI/NI information have this value set to 15 minutes automatically if this value is greater than that.  If it is less than 15 minutes, the system uses the more restrictive of the two values.