Our dog is only alone from about 9am to 3pm, so not as long as the OP suggested his potential dog would be, but I just wanted to point out that all breeds of dog sleep, on average, 13 hours a day. Much of that daytime period that you're worried about the dog being alone, it's sleeping.
This obviously varies from dog to dog and breed to breed, but I personally don't feel that leaving a dog alone for 6-7 hours is all that hard on them, if they are well behaved and you spend the time ramping up the hours in the early weeks of ownership so that the dog doesn't permanently associate your departure with extreme stress and anxiety.
That's been my experience, anyways, with a Wheaten Terrier.
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-Scott
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