They often take a lot of time and skill to mange profitably and can be very difficult to sell when you wish to exit.

@Grace Ng As far as you question on "how long properties stay vacant", you can break that down to 5 other questions.  

I also know of investors that have 30+ units and mange them themselves.

Initially you need to micro-mange your PM while you get used to each other, but once you sort each other out, it is a time saver.

@Albert Ng good idea Albert

Had I not been experienced my most recent purchase looked like a dog with $12K in heating expense, it's been under $3k after a new NG boiler. 

I have a pretty good understanding of were I want to go and will eventually get into multi-units and letting a mang. co. take care of them.

The reason I just recently joined this forum is because I am about to manage my properties on my own, though I have previously manged homes I once lived in myself, I left all the investment properties up to the "professionals"In my experience I found one of two things to be true with prop man companies.

he finds himself to mange the apartments?

It will be significantly easier to mange your investments and your cash flow.