10 days is the max (except for some UK packages that include longer tickets for their typically longer stays). Beyond that an AP is a better deal IF you will go to the parks every day.
We often go for 2 weeks in Aug/Sept and plan a few non park days for rest and for sanity. We enjoy the pool, play mini golf, maybe see the outdoor movie at the resort in the evening or shop and eat at Downtown Disney. Planning a "late sleep in" day does recharge the batteries for more park days.