By default, the vCenter web UI (Flash/Flex or HTML5) will kick users out after a certain period of inactivity. Usually, at least in my experience, users get kicked multiple times per day. Here's how to extend that timeout.
session.timeout = 120
Note: This value is in seconds. For example, a value of 600 equals 10 hours.
session.timeout = 600
service-control --stop vsphere-ui service-control --start vsphere-ui
service-control --stop vsphere-control service-control --start vsphere-control