What you are describing is Alert Escalation and only requires one alert, but multiple trigger actions.
Here's the steps:
Create your alert criteria on the "trigger condition" tab
Create your trigger action for alert log
Create your trigger action for email, but on the Alert Escalation tab select "Do not Execute..." and set "Delay Execution..." time.
You can add any combination of these settings to fit your requirements.
Remember: Alert Trigger Condtion only happens once, but you can have many Trigger Actions
HTH
Stuart