Well Zapdos and Moltres took about 2 hours each. But copared to Latios that is nothing. Encountering was easy, but I ran him to minimum HP and to the point where he had to use struggle about 5 or 6 times and still couldnt catch him with 70 ultraballs and 10 timerballs.
I found Moltres the hardest, because Fire Spin was a big pain; it chips away at your health and I couldnt switch out... Also it just wouldnt stay in the 50+ Ultra balls I used!
Articuno was the one I HATED because I would always go in the wrong entrance to Sea Foam Islands, there were always a million wild pokemon, and I almost never bought enough ultra balls so I would have to restart and buy more & do it all over again. Not to mention every time it used ice beam I would get frozen