It really depends why you are going there...if you are going for just the Harry Potter stuff, by all means one day is enough, but it won't give you much time to do anything else at all, especially if you go in the middle of the summer. I know its a lot but my recommendation is to stay at one of the 3 higher class hotels they have on resort because you get an unlimited express pass and early entry. The early entry is only good for the harry potter areas and usually only one opens early per day. The express pass, basically a fast pass you can use all day, isn't good for either harry potter rides but its good for all the other rides in the park. Send me a board mail and I can give you lots more details about it all and the best time of year to go and what not. I went last year with my wife in September and we probably rode every ride in both parks 3-4 times in 2 days. Also worth noting if you want to ride Hogwarts express you HAVE to purchase a park to park ticket.