You will receive every 5th night free on standard room reward stays of 5 nights or more. (uponarriving.com)
Since premium room rewards are fixed at relatively low values and do not offer discounts for longer stays, you should stay away from these reward options in the vast majority of situations. (thepointsguy.com)
All other nights of the stay will be charged at the applicable full Standard Room Reward price. (hiltonhonors3.hilton.com)