Even though you're staying onsite and will have FOTL access, do NOT rush...take your time and see everything in both parks. The parks close at 10pm until the middle of August and if you're going during that time, split up the day and take an early afternoon nap...or swim in the pool before going back in the evenings. Universal is really cool at night!
At LEAST two full days. You can experience each park in one full day (each) pretty well, but it will leave you wanting more. Maybe do a third day to do your favorites again and see anything you didn't get to. If you're planning to take breaks or play at the pools, at LEAST 3 days.