The trial and what follows
Three days from your account’s first publish, covering everything it publishes.

The clock starts once — at your account’s first publish — and runs for three days. It is a window on the account, not a term per app: publishing something new does not buy another three days.
While the trial runs, a Trial version strip with the days left sits at the top of the app and a builder credit at the bottom. A paid plan removes both.
The Publish dialog says so before you press the button: on the trial a note above the tabs gives the term, what customers will see and a Choose a plan button, and for a published app the day it opens until.
When the trial ends the app stops opening, and you cannot publish anything new or update what exists until you pay. Your projects are not deleted — they stay in the editor, and paying puts everything back.