Take the number of days you expect to be in the wilderness and add 2-3.
They don't care if you come out early, but leaving after the end date could raise questions.
Really, the entry date is the only thing that you have to be exact on.
So many variables come into play here though if you leave past your end date. Lack of fitness caused a slower hike. Altitude sickness slowed you down. Weather slowed you down. Injury slowed you down. Fatigue due to loss of appetite slowed you down. If you're exiting and DO have a permit, you are obligated to no explanation to anyone as you are descending a few days later than you had planned.
