That completely depends on when you are going, and how you go. During the busiest time of the year you will need 4 days to see and do everything and your favorite things more than once. During slower times, you will be able to see at least almost everything as long as you are an open to close kind of person. The obvious trade off is weekends are busier in general, but have much longer hours. Fireworks and Fantasmic are weekends only until summer.
Universal Studios is just 40 miles away- but it is an hours drive during the middle of the day or the middle of the night- to get there at opening, you would be driving in commute traffic during the week- allow 2 hours.
Universal can be done in one day- if it is a slow day and you open and close the place. Two days is ideal and their tickets are usually buy a day, get a day...