Our longest was 16 nights with 10-day park tickets. We were suppose to have two days in each park with the extra two days in whatever park we felt needed more time. We ended up 1- AK, 2- HS, 2- Epcot and 5- MK (we had a 5 yo with us).
On our out of park says, which we schedule every third day, we swim, mini-gild, DTD, explore other resorts and just rest. We have found we get too tired and don't enjoy our trip as much if we don't take breaks. One down day for every two in the parks is perfect for our family.
We have also found that around day 10 we start to get tired of eating out. We do stay where we have a kitchen and eat breakfast in most days, and lunch on down days, but eventually we start missing our usual meal time dishes.