My family painted about half our house ourselves when I was about a pre-teen. My mom just hired a painter to paint her new house. The painter didn't take an appreciably shorter time period than a couple pre-teens did. IIRC, it took us about a week for about 800 square feet worth, and it took the professional 3 weeks for 2400 square feet. And yeah, we did the whole masking-tape, dropcloth, and primer routine as well, and our corners were just as good. (I was gonna say we probably took more cajoling, but given the number of times my mom had to call the painter to make sure he showed up and would have it all done by move-in, I'm not sure that's true.)

Painting isn't a profession with a particularly high return on experience. You hire a painter for comparative-advantage reasons. It may take him just as long as it takes you, and several thousand dollars more, but chances are you can make more than several thousand dollars in the time you would've spent painting your house. (If you can't, you may want to re-think hiring a professional, and instead go into business as a painter yourself...)