US Embassy says that
ballots should be received in the Embassy for transmittal through the Diplomatic Pouch no later than October 15, 2020. Ballots received after that date will be forwarded, but may arrive too late to be counted https://ch.usembassy.gov/voting-09-29-2020-2/
(This is publicly available, so I can share it, right?)
We sent ours via Swiss Post on 21 September. USPS (same Swiss Post tracking number) shows that they are out for delivery this morning--just under two weeks.
Someone who votes in another state was able to do it all electronically and it looked so efficient and secure. Wish it was that easy for all of us.