I'd say on average you need a day and a half per park to move at a comfortable pace and see 75% to 100% of what's there. Commando style planning and touring can get you done sooner, as can an EP, but on average, not rushing I think you really need a little more than a day.
On the other hand, heavy crowds, a late start, or no EP during a peak season could take you longer. The parks don't stay open as long as Disney's do, so just to fit in the rides, a few shows, lunch, and any breaks, really takes about a day, unless you rush it or skip a lot. I'd give myself a day at each park if nothing else, unless there were only a few every specific things I wanted to see, like going only for HP.