Content Warning: Discussion of sexual violence.

My general question is: what was life like as a slave in a normal situation? Was it as bad as depicted in movies for MOST slaves?

It's impossible to give you the kind of numbers you might expect in a modern social sciences study; the nineteenth century doesn't have records like that. Nor can we speak of every slave in every situation. But a typical enslaved person? In a word: yes.

Enslaved people suffered tremendously on a daily basis from punishing work conditions, malnutrition, and other forms of constant abuse. If anything, what we see in recent films like 12 Years a Slave may even undersell it. That's before we get into the violence doled out as punishment or just because an enslaver feels like it that way. Even when one isn't being directly tortured by whipping or other means, the threat of them hangs over every enslaved person just as does the destruction of their families through sale. And when it's not a particular enslaved person's turn under the whip, in chains, or whatever, it's someone they know.

How long were the work days?

Before dawn to dusk or later, depending on the season -longer hours for harvest- and assigned tasks. Enslaved people processing sugarcane can expect to be up for at a day or two without sleep because if the cane isn't all handled within that time it's a loss. To most days, you can also add a few hours afterwards where they work on individual plots to supplement their diet, do maintenance on their quarters, and other stuff that isn't their official duties but needs doing.

Was sexual assault common?

Very. Sexual violence against enslaved women is so common that there's an established trade in "fancy slaves" who men buy with the intention of raping them repeatedly. High society white women would gossip about how all the mixed-race kids in someone else's house looked like the owner. Male children, relatives, and overseers are frequent culprits too. Sometimes fathers and sons afflict the same woman in turn. Rape is so common that it and the fear of it are a major theme in slave narratives written by women. Even the men admit it, to the point of arguing that the availability of black women to rape is a benefit of slavery under the theory that rape will happen anyway so better it not be white women raped.

This is Benjamin Franklin Stringfellow laying it out:

Negro slavery has a further effect on the character of the white woman, which should commend the institution to all who love the white race more than they do the negro. It is a shield to the virtue of the white woman.

So long as man is lewd, woman will be his victim. Those who are forced to occupy a menial position have ever been, will ever be most tempted, least protected: this is one of the evils of slavery; it attends all who are in that abject condition from the beautiful Circassian to the sable daughter of Africa. While we admit the selfishness of the sentiment, we are free to declare, we love the white woman so much, we would save her even at the sacrifice of the negro: would throw around her every shield, keep her out of the way of temptation.

That's not a private document written for a few friends. Stringfellow penned it as part of a wide-ranging proslavery pamphlet meant for general circulation in defense of his and his buddies' plan to invade Kansas. He wrote it, circulated it among his group, and they voted that it should be published.

Did they get any days off, ever?

Sometimes. Custom gave enslaved people Sundays off, Christmas, and maybe half of Saturday if they were lucky. All of this goes right out the window of the enslavers want them for something in those times. The respites don't generally suffice -nor are they meant to- so the most common reason that enslaved people ran away from their enslavers was probably to get some time to themselves and visits distant friends and loved ones. Unless you live near to a major transportation hub or a free jurisdiction, running to freedom isn't usually a realistic option.

How early in a child's life were they expected to begin hard work? Did they get lax treatment compared to adults for a period of time?

It depends on the individuals and the desires of the enslaver, but children were mostly left to themselves until around six. Then they would begin with light duties that steadily increase until they're around twelve or thirteen. Demands increase right about the time, or a bit before, that the child is physically and mentally able to handle the activity.

How difficult was it to get in a situation where the owner would physically abuse you?

An enslaved person, simply by being enslaved, is in that situation. Slavery is an inherently violent system and cannot be sustained without regular brutality to keep the terror alive. Enslavers who refused to whip people, or did so sparingly, earned the ire of their peers for being irresponsibly light touches. One can be abused for any reason or no reason at all, without hope of appeal.

Was it common for slaves born into that life to think of themselves as lesser humans?

No. Any form of persistent abuse brings with it feelings of shame and hopelessness, as well as a realization of how much is denied to you, but I wouldn't say that in general enslaved people commonly internalized white supremacy that way. That was what white people did. Remember that the enslaved are in daily contact with whites. They can see just as well as you and I the range of ability and talent that exists among them.

Did the slaves on plantations occasionally riot, like in a prison?

General violent uprisings are pretty rare in the US. The enslaved know the odds are very much against them if they launch a general insurrection. That's not to say that no one ever did rise up, even with a general plan to launch a revolution, but for the most part the whites hold all the cards. An individual might get fed up and lash out at an overseer or enslaver violently, but resistance tended to be more in the form of quiet sabotage. Enslaved people would break tools, slow walk tasks, deliberately misunderstand instructions, abuse livestock, and otherwise make nuisances of themselves to eek out a bit more freedom. It's a significant enough issue that enslavers end up buying extra-rugged (and more expensive) or very flimsy (and cheap) farm tools because they expect the slaves to break them.

A number of these questions are answered in this AMA.