It depends on what you want to see and who is in your family. If you are traveling with small children, you will be interested in different things than if you are traveling with teens or just adults.
With little ones, we do 2 or 3 days in magic kingdom, 1 or 2 in epcot, 1 in animal kingdom, 1 in mgm. Of course, you could do more or less depending on how much you want to see.
You do need at least one full day in each park though.