I have no idea if there are default check-in/out times, but YOU can set your times. Where you do this depends on what version of the AirBnB interface you see in your browser. (Apparently, they're currently rolling out some changes and different hosts are seeing different menus, etc.)
For me, the old "Terms" category has disappeared. I now find the settings for the Check In/Out times and my cancellation policy under the Check-in page, accessed by: Open your calendar. Click the unlabled button that has four horizontal bars, found near the upper left above your calendar. When you click that, a menu will slide out from the side and you'll find a Check-in link. The rest should be self-evident.
If you find this helpful, consider clicking the Thumbs Up button near the lower left. Thanks!