I now have a dog door (and am retired) but when I got my first greyhound I was gone at work for 9-9.5 hrs five days a week, plus one day a week my schedule was 12-9 instead of 9-6 with no dog door. She was just fine with that schedule, never had an accident. After that, whenever I got a new dog I would come home at lunch for the first week or so and let them out. After a week, or sooner, when I came home they would look up from their beds and say “why are you disturbing our nap?”
Generally, if they are healthy, anything up to about 9 hours should be fine. Obviously, if there are health or behavior issues, or you have an elderly dog, things could change.
Good luck!