Can I create a trigger to send an email if an incident has been on hold for so many days?

Jan is right.
First, you need to create a custom date field under the Service Desk Settings. Before you do this you need to decide if you want this global across all services or just to one specific service. If you want it global, create it under 'Account-Wide Settings' otherwise create it under 'Services' tab and select the service you want:


Next, you need to create a trigger that populates the data field you just created every time the SLA clock is put on hold:

Now you can create your report to display any incident that has been on hold for longer than 7 days:


Make sure you save your report so you so you get access to 'Schedule & Queue' in the report toolbar. Select 'Schedule & Queue' then add a tick next to 'Action Schedule'. Add a task to 'Add a comment' to the incident and set it to run once a day. Make sure you also add a tick next to 'Watchlisted', to notify them:


This should get what you want.

You could add some tweaks if you want so it doesn't keep emailing the same result every day. Let me know if you need help with this.
 

If you needed to, you could also create a custom date field that gets populated by a trigger anytime the incident gets changed to the "Hold" status.  Then use your Status field and the new custom field as your criteria for the report.   I'm just throwing some ideas out there for you.  Hope this helps.