Universal Studios and Island of Adventure are 2 separate parks. The Harry Potter area is part of Islands of Adventure. Compare it to say...Epcot, and Magic Kingdom, with Fantasyland being a section of MK.
You can buy 1 park tickets, or tickets that allow you to park hop, just like at Disney.
Cant help you on the amount of time it will take. We are heading there for the first time in April I'm planning on 4 days, since I know it will be crowded.