At the bottom of every reservation made directly is a link to
https://www.hilton.com/en/hi/info/re...on_rules.jhtml which states :
Check-in/Check-out Policy: Rates quoted are based on check-in date and length of stay shown. Should you depart early prior to the departure date confirmed for any reason, hotels may impose an early departure fee.
I've always found that early check-out is generally based on the same conditions as the cancellation policy for your reservation. If your booking has a "6pm day of" policy, then you're good to just leave on any day (before the checkout time). If it's a "day before", then you need to let them know you're leaving the day before. If it's a 3-day cancellation policy and you let them know the day before then you're probably going to be hit with an extra night charge or a fee (but not both).
Of course, as always this is at the hotels discretion, and thus can be based on status as well as how frequent a guest you are at that property.