If you want to see how long a unit has been occupied or vacant, you can look at the Rent Roll report in WelcomeHome!
The report will default to having additional columns shown, but if you just want to see information related to days occupied/vacant, click on "Edit Columns" and only select the Unit, Days Occupied/Vacant, Move In and Move Out columns. Click apply, and you'll see the unit number and how many days it has been occupied or vacant.
The Move In/Move Out columns will help you see if the unit is currently occupied or not, which will let you determine if the days is related to days occupied or days vacant.