Unfortunately, nobody knows. We do not have slavery statistics for Viking-age Scandinavia. We know there must have been slaves, but our best information is either very circumstantial (unconventional burials, for example decapitated bodies placed near or on top of another, presumably the owner), or from literary sources either far away (Iceland, Frankia) or long afterwards (the 12th/13th century law-codes). Based on these, it is probable that slavery for Vikings followed a similar model that it did in the non-industrialised areas of the rest of Europe, as well as in antiquity. We have no evidence for the big slave estates of the Romans in this period, and literary evidence also suggests slaves were mostly household helpers (servants, the latin word 'servus' is the most commonly used). The Scandinavian word 'thrall' has associations with hard physical labour, so someone who is a thrall is someone who does the nasty jobs, basically. All this suggests that slavery was of a relatively modest scale (compared to, say, imperial Roman Gaul, or the Antebellum South). Slave shackles (as you would find in the aforementioned areas) are also absent from Scandinavia, and if you read the literary sources it appears that slaves in principle were not physically restrained from leaving/escaping. This all suggests that slavery was primarily a social status that you carry, and that your social relations was what kept you from escaping.

Unfortunately, we know nothing about the details, such as whether slavery was heriditary, or whether it was customary to be released from slavery after a certain number of years (as was common in Islamic household slavery), or whether the release of slaves in your will or testament was seen as a good act (as it was in Christian Europe).

It is possible that we have so much difficulty seeing slavery in Scandinavia itself (in contrast to the regions around it, from where slaves were taken or where they were sold) because the free/unfree distinction is different here. It is possible that slavery was just one (or multiple) kind of being dependant, just one strut on the long social ladder running all the way to the king. The distinction between having a right to carry arms (and be a legal actor on your own, a family representative so to say), and not having that right (a bit like the Roman system of clientage) was something that was far more important to emphasise in material displays of status (like in grave furnishings, for example).

Our best documentation of early medieval slavery comes from England, which increasingly fell within the political and cultural purview of Scandinavia during the late 900s and early 1000s. In 1086, William the Conqueror took a tax inventory of his new domains, and this so-called Domesday Book recorded that 10-30% of his subjects were living their lives as slaves, depending on what part of the country they lived in. This gives us a fair baseline for assessing what percent of contemporary populations elsewhere were enslaved.

Viking Age Scandinavians didn't do many of the things that we associate with slavery. They didn't have plantations, mines, or monuments. They did have a constant need for rowers, but the clinker-built ships of the North Atlantic were no place for the galley slaves of Mediterranean triremes. These factors set the conditions for a "minimalist" view of slavery, so I'd guess about 10% of Scandinavians were enslaved during the Viking Age. (Incidentally, later laws would prohibit ship captains from having slaves make up more that 10% of their crew.)

In the absence of heavy industry, most slaves in Scandinavia probably worked as household servants. Large households might have a few dependents who were considered slaves, and even small households might have one or two. They probably did the dirty work of mucking out the animal stalls during the winter and manuring the fields during the spring. There's also evidence that they made a lot of the day-to-day pottery for household use. It's likely that the children were constantly spinning thread and the women constantly weaving. Men would work the fields through the summer and into the harvest, when women would be busy making cheese and pickling food for the winter. In short, they'd be engaged in many activities side-by-side the free members of the household that they shared room and board with.

Extraordinarily wealthy slave owners (for example, whoever buried the Spillings hoard had enough wealth to buy 100 slaves) might even settle entire families of slaves to cultivate outlying fields. Based on the lack of ancestral cemeteries associated with these small farms, it seems that these families were unable to bury their dead in any special way and inherited their unenviable status. There's also the question of concubines, but practically speaking, they doesn't seem to have been common. Attractive slaves could cost three times as expensive as regular slaves, and the wife of a household might refuse to have concubines under her roof, forcing her husband to independently supply his concubine with an outbuilding and all the necessities of life.

Textual sources give us two further insights. First, they tell us that Viking Age slavery was a legal status that didn't necessarily match up to a person's social status. Some slaves could be very well off, and some free people could do very poorly. Second, they point to the importance of the slave trade. Missionaries traveling from Francia and diplomats from the Middle East were similarly impressed by the volume of Scandinavian slave markets. If the average farmstead had a small number of slaves who had either been purchased or inherited their status, then the proportion of slaves in trading towns must have been much higher. For example, one Russian merchant who met an Icelander somewhere in Denmark or Norway traveled with twelve slaves ready for sale. (This would not much skew the overall statistic of about 10%, however, since only a very few settlements numbered 600-1000 residents.)