I think that the ideal time for the parks is 3 to 4 days, one park per day, USF first, then IOA next, then back to the best of both parks on day three.
For instance, in my May trip, I did a half day in USF on day one, we got there at 2pm, a full day of USF on day two, a full day of IOA ending with USF at Curious George, then on that Monday we came back for a half day at IOA, early entry, lunch at Confiscos, and ended the day at USF. This covered the full matrix of everything we liked, two or more times on the rides we loved, and more time at the play area for my son, who could probably stay there all day!
It always depends on your family and whether or not you have FOTL, but with or without it you will not lose with at least 3 days.
Have fun!