All response above are valid - I'll add my experience: I currently have two units.
I spend on average, about 2 hours/mo. on bookkeeping, tracking expenses, running P&L's, etc. That is on my own time, whenever is necessary. I have trained my tenants to email me instead of calling. I answer about 2 emails a month, and I check in with them at least every 60 days if I don't hear from them. (I also have it in my lease that all repairs will be dealt with during business hours, if there is an emergency they need to call the fire dept. not me.)
I have had maybe 4 repair calls in the last year and a half. I take care of it when I have the free time, or I just call a plumber/electrician/etc. (That's what a PM will do, plus likely up-charge you.)
I do ~30 minute inspections each quarter on both houses, plus travel time of ~45 minutes = 45min/mo. average
Leasing each unit took about 12 hours total of my time - posting, phone screening, open house, signing leases, and dealing with HA's. Note that mine are S8 tenants, so if you are market rate your time could be much lower.
Okay, so let's do some math:
Let's say 3hrs/mo of bookkeeping, inspections, calls, emails, etc., and 12 hours every time I lease up.
My avg rental rate: $1700
10%*2*1700 = $340, @ ~3 hours of work = $113/hr
Leaseup fee $1700 / 12 hr = $142/hr
Granted, I do spend more time turning the property, checking on contractors, etc. But a PM would also charge me for that activity so I'll keep it separate from this discussion. I would guess the hourly rates are similar.
Now the question:
Do you want to pay yourself >$100/hr, learn the ins and outs of the management side of things, and take care of your own properties, or do you want to pay someone else to do that?