RentHQ has a new report called Vacancy Days, which lists each property, and prints the number of days it has been vacant as well as a total vacancy days percentage. You get to choose the date range to analyse. This report is found under Analysis Reports on the reports screen.
This is great for finding out which properties are under or over performing.
The report automatically excludes any property which has never had a tenant. It also ignores the time prior to the first tenant moving in. For example you may choose to run the report for the last 10 years, but one property was only acquired 1 year ago. The report is not going to say that property has been vacant for 9 years.