How long does it usually take to fill a small apartment building?

[Kaleb DeCook]

I'm looking at buying an 18-unit apartment building that's recently been renovated and hasn't been advertised for rent. I'm trying to estimate how long it will take to get rented out with a good property manager. It's in a small town with a population of just under 3000. There's not much currently listed for rent in the area. Thanks for any input.