An alert would show on the dashboard only when it meets the set criteria and get triggered. All alerts generated by you (but not triggered) would not show on the dashboard but in the portfolio alerts set-up screen.
If a portfolio alert is due to expire (i.e. it has not been triggered for 90 days), you would get a notification message 5 days prior to expiry of an alert. At the same time expiry date is displayed with hyperlink in red color - this hyperlink facilitates you to renew the alert.