8 hours is the maximum amount of time an
adult dog can be reasonably crated. So it depends on the age of dog you're asking about

If it's an 8 week old puppy, for example, then no you absolutely *couldn't* crate her for 8 hours solid.
The reasons aren't about exercise (though yes, absolutely, significant amounts would be needed after that amount of sedateness). The reason is health. No animal - not dog and not human either - is designed to hold their bladder for extended periods, except during deep sleep (i.e. night time) when their bodily functions slow down or cease. If it's not clear, what makes night time an OK time to go 8 hours without urinating is that the body isn't producimg urine in the first place (or at least, only at minimal rate).
But during the day time, urine production is full steam ahead. And it NEEDS to be full steam ahead, because this is the means by which the body rids itself of toxins and bacteria.
If you don't drink adequate amounts of water, and don't urinate every 2-3 hours as nature has designed you too... well, then your body isn't able to expel the toxins and bacteria that build up during the day. And your health will suffer as a result - you stress your kidneys, which in turn stresses liver, heart and every other internal organ that relies upon the urinary systems for removal of waste (which is actually all of them

). You will also put yourself at risk of kidney and bladder infections, crystals and eventual stones. That being because not urinating often enough goes hand in hand with not drinking enough, and the two things result in urine concentrating in the bladder - rather than being expelled frequently in dilute form. It is only in concentrated urine that crystals (leading to stones) can form. And of course, concentrated urine is a breeding ground for bacteria. This is why doctors advise us all to drink 2 litres of water each day

It is for avoiding these sorts of problems. Of course, doctors don't need to tell us to ensure that we urinate every few hours - nobody is going to hang on in discomfort rather than simply taking themselves to the bathroom as the need arises.
Well - the above all applies every bit as much to dogs. Same needs, same health impacts if they're denied adequate water and opportunities to urinate (or don't drink adequate water, because they already need to pee but can't).
Now of course, we don't live in an ideal world, and most people have to go out to work all day. So it isn't possible for us all to provide potty opportunities for our dogs as often as they should have them (which is every 2-3 hours). So we do have to accept a compromise that is less than ideal. But less than ideal shouldn't extend to the point of inhumane

There are limits to what is reasonable to ask, and those limits depend very much on the age of dog (due in part to bladder size).
A general rule of thumb for the MAXimum amount of time you can ask a puppy to hang on between potty stops is the age in months, +1. So a 2 month old puppy must not be left more than 3 hours. A 3 month old no more than 4 hours. And so on. Once you reach 7 months/8 hours - that's the maximum. It isn't reasonable to ask any dog of any age to hang on for longer (not just for discomfort reasons - though I'm sure it would be extremely uncomfortable - but for the impact on the internal organs).
And I'm sure you can easily see that the more often you ask this of the dog , the greater the impact, especially long term. It isn't going to kill anyone (dog or human) to have to hang on once in a while. But when it's a daily thing, then the stress on the organs is going to have an effect.
So in short - keep in mind what is reasonable and what isn't for whatever age of dog you're talking about. If you must ask them for the maximum amount of time reasonable - well, you must. But if you don't *have* to - then it is very much better not to.
If it happens that you're unable to come home often enough to provide the frequency of potty stops your dog requites, you would need to either: (a) NOT crate the dog, and provide some other potty alternative such as newspaper on the floor; (b) find or pay someone else to provide the potty stops (friend/family member/neighbourhood kid/hired dog walker); or (c) look into doggy day care.