Long term rental - rules of thumb?

[Tyler Solomon]

What does a typical spread, % wise, on a healthy long term rental look like between gross annual income, and net operating income? In other words, what is a typical/ decent/ acceptable margin between your annual rents collected and your total year end profit on a long term rental property? Assume there is no property management company in place. 

What are other good rules of thumb you have picked up or heard  for long term rental investing?

Thanks!