We were there last week, on Thurs and Fri. We went from open to close on Thurs, and did all of one park, and all but 4 or 5 things, and the interactive wand stuff in the WWOHP in both parks. Returned Friday to do the wand stuff, the rest of the rides, and re-rode about 5 of the rides, and left by 3ish. We stayed on-site at HRH, so we had the Express Pass.
2 days should be more than enough, with the Express Pass. Depending on crowd levels, I guess. When we went, crowds were light. Even the most popular rides only got up to 60 min wait. Which isn't all that terrible.