When is it cost effective to have a Property Manager?

[Douglass Belt]

Is there a "general rule" for the number of units where it makes sense financially, to use a Property Manager (PM)? For instance, 5+ units use a PM? I am looking into making the move from a SFH to a Fourplex. I understand there are numerous variables to justify the cost, however, I wanted to get thoughts and experiences from those who use a PM and are in small residential investing (2-4 Units). Is it worth the expense? I enjoy real estate investing, however, my concern is that I am worried that increasing the number of units, will also increase my stress (of course that will be a given regardless) level should I manage it myself. Am I correct that about 10% gross rent is the typical fee for PM's? Do you set up weekly meetings with the PM to discuss any issues? If someone is willing to share with me their specific process, and examples of working with a PM that I can learn from, that would be great! Thanks for help!!