i do a split stay each year.
i stay a good amount of time at both resorts/parks.
it depends on the dates you are going and if you are staying onsite that could help determing how many days you need at UO.
if you have a family, it always takes longer to do parks no matter where you are.
when a newbie asks, i always say, 3 days.
you can hit all the highlights, do repeat rides, see the shows, eat at table service places (in park, hotels, city walk), make 1/2 a day a water ride day, spend time picture taking, visit with the characters, etc.
3 days will allow you to explore, take in the ambiance and themes of each section and be able to do it at a leisurely pace.