We went to Universal for one day last year, purely for WWoHP - we went mid September and granted the queues were not as bad as we were expecting. I will give you an idea of what we managed in the one day. Hopefully it will give you something useful!
We managed to do all the rides on both sides - including the Dragon Challenge twice (we thought it was that good
), the Hogwarts Express (twice! Some queuing, but that was to be expected. Diagon Alley to Hogsmeade was about 5mins, coming back the other way was about 20mins) and lunch at The Three Broomsticks, plus a look round some of the shops. The longest wait we had was about 40-45 mins for Flight of the Hippogriff (which, in our opinion, wasn't worth the wait) and about 30-35mins for Escape from Gringotts.
We also managed to do the interactive part at Ollivander's, were a member of the group is picked out by the witch/wizard HP style, "a very strong magic in you" type situation, and that person follows the witch/wizard's instructions by performing spells until the right wand is found.
Out of about 10 kids in the group we were part of, I managed to get picked out!
It is great fun, and, of course, you are offered to buy the wand at the end (which I did!), which you can then use to go around both Hogsmeade and Diagon Alley performing spells at certain points - much to my DBF disgust as he had to stand around while I tried to master my new wand!
After that we managed to walk around Universal Studios taking in the sights and managing a few of the other rides, including The Hulk (twice) - next to no queues, unless you wanted to sit in the front row. The Hulk was a straight walk on.
Ideally, we would consider staying a night or 2 in order to do the full Universal Studios/IoA experience, but we found the 1 day we spent adequate to do the WWoHP comfortably. But again, we did go during a quieter period.
P.S. WWoHP is amazing. Fun for all the family & so much effort has been put in. Even for those not interested in the rides, it really is an experience to see the shops, street performances etc.