I did 2 round trips back to back from LAX-BOS-LAX and 2 more from LAX-JFK-LAX. I don't remember if we got in early or late, but the schedule times were:

UA176 LAX-BOS
8:40AM~4:56PM
UA169 BOS-LAX
6:21PM~9:38PM
UA180 LAX-BOS
10:25PM~6:43AM+1
UA165 BOX-LAX
8:35AM~11:54AM
TOTAL TIME: 27:14

UA4 LAX-JFK
7:00AM~3:03AM
UA25 JFK-LAX
4:25PM~7:28PM
UA14 LAX-JFK
10:00PM~6:03AM+1
UA29 JFK-LAX
7:05AM~10:10AM
TOTAL TIME: 27:10

All my friends thought I was insane, but after I show them our vacation pics to Sydney and learned I only pay $1100 for above to yield 3-Y seats to SYD, they can't wait to join me for the next UA great offer.