Units 489, 491, and 493 are all part of a single property triplex I own. I live in unit 491 full time all year. Unit 489 had 4 months vacancy. Unit 493 was rented full time all year. What do I say for days rented for this property?
If it was available for rent, even if not actually rented, then use the whole year for the time rented.
Hopefully you have entered the 2 rental units as individual units on the Sch E then these days rented questions should be easy to answer. And your unit should not even be mentioned at all. Then all the common expenses should be divided accordingly and the rental portions entered on the Sch E properly.
The only time that "days rented" comes into play, is if you have any days of personal use. If the property was vacant for 4 months and available for rent during the period of vacancy, and you did not use that vacant property for any personal use for one single day of the entire tax year, then it was rented the entire tax year.