I feel like this is impossible to answer. Ideally no dog should be alone every day for a full work day. But most of us (that are lucky enough to be healthy enough to work) will need to have full time jobs. I think most adult dogs that have been trained properly are fine home alone for a work day. They know the routine and allthough they may be bored, they are not anxious about being alone.
Of course this puts more pressure on dog owners. You will need to dedicate the rest of the day to cover the dogs needs. So that means getting up early to walk them before work, sacrificing plans on weekdays because after work the dog needs to be the priority. It also means months of gradually getting the young dog used to being home alone. Making sure you have babysitters and/or people who can help during the day so the puppy is not home alone for longer than it can handle. Keeping in mind that it takes many months before they can go all day without weeing because of their tiny bladder.
There's never a guarantee. And I do think cockers are prone to separation anxiety because of their nature as velcro dogs. My own dog has struggled with separation anxiety, and it was very difficult. I had to use babysitters and dog walkers and spend a lot of time retraining her to be home alone without stressing. But we have also had several years where she is fine, I've filmed her and she snoozes all day until I get home. I think that is what most adult dogs do when their owners are away, sleep.
I'm single and currently working from home. But when I was away every day I never made any plans on weekdays. And never more than a few hours on the weekends. When I came home from work/college I spent the rest of the day walking, training, cuddling, grooming and playing with the dog. It really is a lifestyle, and definitely worth it for some of us, but it does put limitations on social life and other hobbies. I guess you get a little more freedom if you are two though.
So I agree that it depends on the dog and the owner and the support network you have. If you are prepared for the months it takes to get the puppy used to being home alone, then it will
probably work out fine?
I guess my "helpful" answer is that I don't know. For an adult dog I think a good compromise is if you can make it home to let them out for a toilet break during lunch, or have a dog walker take them out for a walk during the day. I did that when I had 12h shifts, payed a dog walker to take her out for a couple of hours mid shift. For normal 7,5h days I just hurried home with a guilty concience