2 at an absolute minimum! I do at least 4 and preferably 5 or 6 at U 3 or 4 at Disney for me. Many people make the mistake of going one day and just doing the old main Universal park. BIG mistake! Islands of Adventure is really the better park in my opinion (although I love both). You have to do a day for the old park and a day for Islands. But its easy to do 3 or 4 as well. The more the merrier. But whatever you do do not miss going to Islands OA.
My suggestion is stay at the Royal Pacific and request a IOA view, probably best view in the city. And the RP is my favorite onsite hotel and it is the cheapest, Portifino also great but more expensive. Universal is the best park in the world in my opinioin. I would recommend cutting into your Disney or Seaworld time if need be and adding more Universal time, trust me you won't regret it.