Plan to be at Islands of Adventure early, realistically atleast 30 minutes before park opening, and as soon as your in make your way to Harry Potter as quickly as possible and expect it to be busy and expect to queue.
The area fills up extremely quick and even early on in the morning it can be too busy for some folk. Personally, I wouldnt go in with a timescale set as to what time you want to leave, as things normally take longer than expected. But as long as your patient and willing to spend a bit of time queuing, it is well worth it!
Another point I feel I want to make, without criticising or digging at your post is that in my opinion, the price of park entry is not worth it if your only willing to explore Harry Potter. Im not sure if you have ever visited Islands of Adventure before, but if you havent I really do suggest you make time for atleast close to a full day at the park. Being a massive lover of WDW I cant hide from the fact that Islands of Adventure blows away anything at any of the Disney parks, It really is superb! I know many people who are Walt disney world veterans who had never ventured over to Universal/Islands of Adventure, but once they have, they love it! Its just a suggestion that I thought i'd bring up to you, and since your paying for the whole park, it may be worth it for you to plan to spend a bit of time at other areas of the park, especially if you already have 9 days at Disney.
Anyway, sorry for drifting away from your question. Hope everything works out and you have a great time!