They open by 6:00am and close when they run out of donuts, which is almost always before 9:00am on weekdays and usually before 8:00 on weekends. Sometimes they stay open later if the regulars are hanging around, but that's rare and you're unlikely to get much anyway. Just coffee and scraps.
The owners also close up shop when they take vacations. There is usually a small sign posted on the door, though not always.
Having said that, I haven't been by there during a time I would expect them to be open for a month or so. Hopefully nothing untoward has occurred. They make a mean donut.