How long can you leave a dog alone for?

I'm asking this question again as I forgot to mention some details.

I'm planning on getting a puppy (German Shepherd) some time around Christmas, which is when I have a break, so I'll be able to spend 1 month and a half taking it to puppy school, getting the puppy familiar with my house, training it etc.

However, once my break ends I need to go to school from 9 AM - 4 PM, though one of my family members will be home by 3. If they can, I will ask one of my family members to take the dog to work maybe once or twice a week so it won't be at home alone for too much of the week and I will also be home all day for the weekends. In addition, i'm planning on leaving the dog in my backyard, which is fairly large and I will also leave it with toys to keep it entertained. Once I come back home, I will be sure to let the dog back inside and take it on a long walk. I intend on letting the dog sleep inside as well so it will only be outside during daytime when no one is home.

So is leaving a dog outside for a maximum of 6 hours, for 4 days of the week too long? I've seen a few people say that they leave their dogs for 8 hours plus. Thanks for taking the time to read and answer!
Added (1). All your reasons are very reasonable, except for arguing that the dog might get stolen, or hurt by predators. I live in Australia, it is extremely uncommon for a dog to be stolen and there are no predators where I live. Just making this clear! And as for digging it's way out, if my backyard is secure enough that can be prevented? Oh and when I go off to university, I'll positively be staying at the same house as in Australia they don't have campus living like in America.
Added (2). I also know that getting a dog fully trained in 1.5 months is not possible, I was implying that the dog would at least have started training.
Added (3). Last update. I'm also thinking about installing a dog door so that the dog may go into the house if it pleases. As for the weather, my backyard has area for shade and shelter if hot or raining. I've heard that plastic kid pools are also a form of entertainment and good for hot days? I'm really trying to do my best to make this as suitable for the dog as I can because I've always wanted a German Shepherd, and I'm trying hard to research before committing to anything. So nothing is confirmed yet!

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