Last year we did a 2 week vacation - 10 days at Disney and 4 days at Universal. For us, 4 days was not nearly enough time at Universal and if I had a disappointment about our trip it was that we rushed through Universal and IOA and didn't get to relax at the RPR as much as we should have. I have always been a Disney lover, but IOA may be the best themed park of them all. Unfortunately, we had to rush through most of it.
This year, we are only taking a one week vacation in Orlando. Last week, we made a decision I never would have imagined possible a few years ago. We decided to do a Universal-only week long trip. Actually, it will be 8 nights, at the Portifino.
Our kids are 5 and 6. Here's the way I see us dividing the time:
2 days at US. Not full days; about 5-6 hours each. The rest of the day can be spent swimming at the resort. One day we will start early; the other we will start in the afternoon.
2 days at IOA. Again, not full days, with the rest of the day spent swimming at the resort.
1 day at Sea World.
1 day at WnW.
1 day spent relaxing at the resort.
1 day as a wild card, e.g., swimming, hitting favorite rides.
Sure, 8 days is more time than you need. But as others have said above, it just means that you take a more leisurely pace. One thing I've learned from our Disney trips is that if you do not take time to relax, your vacation can start to seem more like work.
Anyway, those are my thoughts. I will give a detailed trip report when we get back.