You have eight days to accept, reject or object to an e-invoice that arrives under the commercial scenario. If you do not respond within that period, the invoice counts as not objected to, which means silence amounts to acceptance.
The start of the period is often misunderstood. It runs not from the date the invoice was issued but from the date it was delivered to you, that is, the date it landed in your mailbox. If an invoice is issued on the first of the month and arrives on the third, the last day is the eleventh, not the ninth.
TEON calculates the Last Day column on the incoming documents list with exactly this logic. For invoices that arrive under the basic scenario, the accept and reject buttons are not drawn at all, because a basic invoice is not answered through the e-invoice system; objecting to it takes a different route.