That depends on what park you're in, where you're staying and how you'd get there. If you're at the Contemporary and staying in MK, then 3 hours is plenty of time. If you're at a different park and have to uses buses, it may take a while. We stay offsite and bring our car, so depending on the park we're in and where the house is, we can get to our rental in about 30 minutes from "door to door" (MK takes a bit longer). You also should decide if you want to do a break and come back, or just stay until you're tired and then be done. There are pros and cons to each. For us, it's pretty rare that we actually plan a mid-day break. Normally, if we do a park in the morning, we show up around 10am. We stay until we're done. Maybe 2pm, maybe 3pm, maybe 4pm. We head home with no official plans to return. We get home, shower, relax or whatever. Then if we decide we want to head back out, we do.
To more directly answer your question (sorry if I rambled), on days where we have an impromptu break, three hours is roughly on target, sometimes it's four hours. If we walk out of a park at 3pm and decide to return, it's usually just after dinner when we get back.