Last November I took 4 flightsin Business between Brazil and USA, two flights with pmCO crew and two flights with pmUA crew.

Routes were flow on this order:
GRU-EWR (pmCO)
IAD – GRU (pmUA)
GRU – ORD (pmUA)
IAH – GRU (pmCO)
I´ve select to push the call button exactly 3, 4 and 6 ours into the flight and had these results.
GRU – EWR – 3 hours – no response (at least 20 minutes waiting)
4 hours – 12 minutes (while walking around from one class to the other)
6 hours – no response(at least 20 minutes waiting)
IAD – GRU 3 hours – 35 seconds
4 hours – 1-2 minutes
6 hours – 10 seconds (she was filling empty glasses)
GRU – ORD 3 hours – 30 seconds
4 hours – 1-2 minutes
6 hours – 4 minutes
IAH – GRU 3 hours – 20 seconds
4 hours – no response
6 hours – no response

Always heard CO was better, but Tilton did a very good job getting the UA soft product up. Not sure what to say, but from what I understand CO crew tend to disapear during flights.