Our mail-in ballots took six days to travel across Watertown mailed 8/11 arrived 8/17. Expect more USPS slow downs as the November election approaches. Trumps plan to Kneecap the Post Office https://www.politico.com/news/2020/08/14/obama-trump-postal-service-usps-395270 seems to be having the desired effect.
Check the status of your mail-in ballot here: https://www.sec.state.ma.us/wheredoivotema/track/trackmyballot.aspx