Your property is deemed rented the entire year. Just because a unit sits empty for a period of time, does not in any way change it's business use. It's still 100% business use. Provided of course, there was no personal use during that period of vacancy.
For days rented, go ahead and select the whole year. Vacant periods between renters do not count against you if there's no personal use. Generally, you need a week, sometimes quite a bit longer to get the property ready for the next renter. Once ready and so long as you start advertising it for rent once it's ready, you're good. Especially if you actually do get a renter in it - even if that happens in the next tax year.
When dealing with a duplex or multiplex unit and treating it as a single asset on the SCH E, you can run into big problems with the math when you have personal use of just one of those units, or any space within a unit. It can also create a nightmare if the owner decides to move into one of the units, when it comes to the manual mark and math required to get things right on the tax return.
