There is no rule, but there is no good cash flow deal without quality management. Some people buy a 100 unit complex and manage it themselves (not recommended) and others hire property managers for the other side of the duplex they live in.
What we have done is built our own management company in the Kansas City area instead of outsourcing it or doing it ourselves.
I don't know the San Antonio market but elsewhere offsite property management can often cost around 10% (although I don't like the fee structure as it often creates poor incentives.) And onsite managers are usually closer to 6%.
I would be more concerned about getting a really quality manager and don't be afraid to pay a lot for one as the price you pay for the management is nothing compared to how much money you will miss out on or lose with even mediocre management. I've always felt like good property managers were underpaid for how hard their job is but finding a good one is the hard part.