You have to define what your time is worth/hour and what opportunities you're missing out because of dealing with X, Y, X of real estate. For some, time is money, and if that's true, the Time Value of Money principle quantifies things, and personally I think it's one of the best ways to quantify your personal time, especially for a quant-nerd like myself.
I'm a millennial, earning a decent W-2 wage at a Fortune 500 firm with great opportunities in Des Moines. I've flex time off, which is nice - as long as I get my job done, I'm free to do whatever. HOWEVER, there's still perception of how I ought to spend my time in a corporate setting, which isn't ideal. My real estate side hustle is a full-time job in itself but I'm funneling all I have besides de minimus 401k match, ESPP, etc. as those are "throwing free money off the table" if I don't at least match into real estate.
From my experience, in order of most important/expensive:
1. Time/opportunities lost (it's a zero-sum game).
2. Ignorance of a subject due to unawareness/no knowledge of (think tax strategies/business strategies/automation strategies).
3. Select CAPEX expenditures (a roof was $5.57k recently but we managed to layer it into the PA Addendum, where we borrowed on the purchase price at a 6.5 to 1 ratio, and a furnace went out for the same property during the Polar Vortex but we were super fortunate to have home warranty on it (1st year usually applies)). Yes, you can depreciate those items based on certain schedules but money today is worth more than money tomorrow, and yes, those tack on value to your property but it's the TVM/opportunity lost today because of the cash outlay.
4. Vacancies.
5. Difficult tenants.
6. Additional stress produced by this side real estate hustle. I could be climbing mountains. Ties back to point 1. Recall, 10 years from now, my knees/physical self wouldn't be able to enjoy as much as I do today.
My 2 cents. I am self-managing all the properties now but definitely will outsource it once my personal TVM is $100/hour. Currently, I value it at about $60/hour. But, I do enjoy the property management aspect of things, so don't mind learning the bolts and nuts before I pass it on to someone else. ;)