Relax. I never hear back from most of my guests after initial contact at the time the reservation is made until the day before their arrival. Airbnb sends out lots of reminder e-mails to guests before a trip, and it's easy for a guest to mistake those for your messages and ignore them. Generally guests start to get interested again in the details of their trip a day or two before the arrival date. That's a good time to contact them to ask for the arrival time. In extreme cases, I send a message to their cell phone number and that gets their attention.