How long should a house be vacant in Memphis TN?

[Fred Dray]

Reside in CA and considering buying a house for rental income in Memphis.   I'm using the following link because that management company is used by the agent I'm dealing with.

http://memphis.craigslist.org/search/apa?is_paid=a...

Those rentals are the type houses I'm considering and seemingly in nice areas.  My concern is most have been vacant for more than 30-days. 

Is it normal for rentals in that price range and in those areas take more than 30 days to be leased out?  If so, how long does it usually require to find tenants?

Thanks in advance for taking time to share you experiences/knowledge with me.