Hello Choixocku27
If you purchase the Swiss Travel Pass Flex with the first day of validity on 26 July, you can use it at will on either 8 or 15 days between 26 July and 26 August.
Does that answer your question?
https://www.sbb.ch/en/tickets-offers/tickets/guests-abroad/swiss-half-fare-card.html
→ I, personally, do not recommend the Swiss Travel Pass Flex for less than 15 days. In my opinion, a 4 or 8 days Swiss Travel Pass Flex is not useful but pricey. I would rather suggest the Swiss Half Fare Card plus individual tickets or (Saver) Day Passes.
Kind regards,
Roland