I work a full time job with a schedule that fluctuates from day to day. I currently manage our entire portfolio of 22 properties. I have actually found it to got easier as we aquired more properties:
-Less stress: because if one water heater, furnace, whatever breaks, or someone doesn't pay, it is only a small fraction of total income, not 25% or 50%.
-Easier to use the volume to entice subcontractors: plumber, electrician, etc. are quick to respond to our needs because we give them a fair amount of work. Also, at some point you will be able to hire a handyman to take care of all mx requests. Where we are at now, I basically just take phone calls and make a decision on who to call.
-Better systems in place: as you grow you will naturally be forced to put systems in place that will minimize workload.
The next step for us will be to have a full time property manager take all phone calls and complete all showings. I am having a hard time giving up those things right now, but working toward it. Steve L. had a good post about scaling a business up large enough to have a full time employee, he said it took about 60 doors.