It depends on if they're inside or outside and what their situation is. If they’re outdoors in a kennel where they have shade, food, and water, they’ll be fine for quite a while.It’s very easy to let them stay in that kind of a surrounding for a day. I do it almost daily with my own dogs, dogs that live indoors.
From a temperature standpoint, they can handle all day by themselves. The problem comes in with the water, what if they spill their bowl or those kinds of accidents? Do they need to eat throughout the day? A lot of smaller, toy size breeds need to eat more than just between 8:00 and 5:00 or 6:00 PM. And then you have to consider the elimination side or bathroom breaks. You're probably going to walk into a mess if they've been alone for any more than four or five hours, depending on the breed of puppy.
My rule of thumb that I was always told years ago was that puppies can hold their bladder for one hour longer than their age in months. That's theoretical and there’s no exact science behind that, but that's what I was told by behaviorists and it holds pretty true to form most times.

It definitely can be done. That’s why it's important to have a safe and secure place where you can kennel your puppy throughout the day.
If you have the ability to go home and check on them over lunch, even better. That's better for them, but that's basically what it takes. Give them the chance to use the bathroom just before you leave, put them in their proper bedding with things they can't chew up, destroy, choke on, etc. Try to check on them once if not twice during the day to let them out, clean up, and just to make sure they're not getting themselves into anything that they shouldn't

Think about what a dog could get into. You’ve got to think of it from that aspect and perspective, like the viewpoint from being a couple of inches off the ground like that of a young puppy. They're going to see what's in front of them. They're going to find food on the floor. They're going to find trash cans that they can reach up and knock over. They're going to find anything they can get their mouth on and try to chew it. So you might want to invest in a nice kennel or crate—some place that you can put them in. And not as punishment, but let that be their normal domicile when you're not there. So at night, when you have company over, when you're not home directly supervising, the kennel should be a good and comfortable place for them to go.
You might also want to consider depending on where you live and how it's laid out, maybe some baby gates that will prevent them from going to various areas of the house. Those are probably the most basic common sense things I would think that you would need to do first.