Do the numbers. If you have a building earning $3000 a month or $36000 a year, paying a property manager 8% is like $2880 per year. If you work 10 hours per month on each property (which is alot-this time figure does not include any maintenence work) then that time works out to $24 per hour. Is it worth $24 per hour to farm out?
It will take a lot of pressure off of you if you have a professional service taking care of your property. Each company charges a different amount so it's best to check with them directly and compare what each one is offering. It is usually a percentage of the rental fee .