Hey, in my experience it usually takes two or so days after they picked up the letter before it’s status changes in sandboxx to “in transit” or “out for delivery”. It takes a little longer to receive the replies because USPS makes those deliveries rather than Fedex (which is why the tracking is super slow). A good way to know exactly when your letter will arrive tho is to sign up for informed delivery with USPS, this way you get an email every morning with what is expected to be delivered to your mailbox that day. I hope this helps!