It really depends on when you go (a busy season or not) and how much you want to see and do and at what pace.
My brother and I usually spend ONE day there, starting at 9am, and usually hit all of the major attractions in BOTH parks and are out by 5pm with time to spare and spend at Citywalk if we feel like it (grab dinner or a movie). Now we don't tend to "stop and smell the roses", but we aren't running through the parks either. In case you were wondering what I meant by major attractions, this is what we can typically see starting at 9am and going to around 5pm:
IOA:
Hulk
Spiderman
Dr. Doom's FearFall
Popeye
Dudley-Do-Right Falls (Spelling?)
Jurassic Park River Adventure
Jurassic Park Visitor Center *(we usually eat lunch here)
Dueling Dragons (usually only one side)
Poseidon's Fury
Cat in the Hat
We can usually do the above by 11:30 to 12:00, given the park doesn't really get busy until 11am and we go over Christmas when most people opt to not ride the water rides (All of Toon Lagoon at JPRA) because its too cool outside.
Then its off to Universal Studios. By now the parks are getting busier and thus it takes longer to do things here because of the lines:
Twister
T2:3D
Earthquake
Jaws
MIB
ET
From this you can see we skip some things, and with two new rides opening by next month (Shrek and Jimmy Nuetron), we would need more time. We also don't tend to watch shows at Universal. We don't really like any of them

Also, to do the above, as we do, you really need to use their Express Pass system (similar to Disney's Fastpass) to the best of your ability.
Thus, if it is your first time, take TWO days. I don't think you would need more. You can visit Citywalk on any night of your trip if you want since its free to get in there (although there is a parking fee).
Good luck with your planning
*Note: Should you plan to do both parks in a day, you will need a 2Day park hopper pass. The days never expire, so if you use one, you can keep the other till your next trip, as long as you don't take the 3rd Day free offer (then the pass expires in a week)