The first 48 hours are free
The free period starts the first time that wallet opens the panel. It lasts 48 hours. During that time you can publish sites without choosing a plan, up to the usual limit of eight sites for one wallet.
The clock is kept with the wallet on the host, not in the browser. Clearing site data does not reset it.
When the 48 hours end, new sites need a plan. Updating a site you already published is still allowed, so a live site is not taken down because the trial ended.
Plot, Grove, and Canopy are the plans after the trial. Each one is a flat monthly price and a fixed number of sites. There is no meter on traffic or builds.