This is configurable as part of the vSphere Web (Flex) / H5 (HTML) Client configuration
Web Client - /etc/vmware/vsphere-client/webclient.properties
H5 Client - /etc/vmware/vsphere-ui/webclient.properties
The default is 30 days
# The number of days before the notification about expiring password appears.
sso.pending.password.expiration.notification.days = 30
You'll probably need to restart the service for the change to take affect
