BNB customer service is open 24 hours a day.
But I don't think perhaps that is what you mean.
If you mean what time can you arrive for the accommodation you have booked and what time you will need to leave - then each host sets their own arrival and departure time.
You will need to look at the listing/s you are interested in and it will show you check in and check out times.
Mostly hosts will have an afternoon/evening check in and departure in the morning.