I usually try to set a one-hour window for arriving guests to know when to expect them. For people coming from overseas I usually ask for the airline and flight number so I can check to see if the flight is delayed if they don't show up when anticipated. My listing gives a range of checkin hours they should see before they book too.
In the rare cases where there hasn't been a good response or any response about check in time I go about my day. You shouldn't let other plans go as you'll resent the guests. I just send a message saying I didn't hear from them about a checkin time and I'll expect to be back after x o'clock. If I have their phone number sometimes I'll text that too.