It took me ten months to finish Factotum. That explanation is far too long to be recounted. So I won't.
Factotum is essentially a cross of Shakespeare and "Hobbit-Era" Tolkein. The protagonist is so young that it's considered a young adult novel. That's pretty much the only reason I can see for it not being adult. My vocabulary is pretty large, but I couldn't make heads or tails of a good amount of words. However, most of that is because they haven't been widely used since the eighteenth century...
D.M. Cornish's ability to reflect the equivalent time period language in his prose is extraordinary. He had to have done a LOT of research. Above all else, it all felt seemless.
Nothing bad can be said about any of the characters or the setting. The plot had a few minor problems in pacing and felt a little erratic at times.
Overall, I give Factotum an 86/100.