Episodes in Drip feeds of Powerhouse plans have the option for "Drip Expiration". This is the number of days the listener can listen to an episode after it's dripped to them.

In the bottom-left of your dripped episode, you'll see the setting shown below. Select "This many days after initially dripping" then enter a number of days.

In the example below, episode 5 will be released a week after episode 4. Then the listener has a month to listen to that 5th episode. On that 30th day after it was released, the episode is removed from the listener's feed.