It is 28 days because Search Console data settles over several days, weekday seasonality needs whole weeks, and Google takes time to recrawl and reprocess a change. Anything shorter mostly measures noise and would produce confident nonsense.
Before it closes, the product will show you what has happened so far and will always say 'so far'. A number on a screen with no hedge beside it is read as a decision, and clients act on it.
The number itself is a decision that has not been validated against real data yet. It is reasonable for click-through rate and may well be short for position, and it is written down as an open question rather than presented as settled science.
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No card, and the free tier does not expire — the first verdict takes about six weeks to arrive.