I don't know. Like, would I find it less offensive if it was used five or ten times as opposed to 110 (apparently the number of times its used)? Possibly. But it's more about how it's used. Every character in the movie uses it but (tellingly) Tarantino, including the black characters towards each other; and sometimes the context shifts from historical to modern and back again. I think it has less to do with how often it's used and more to do with why it's being used so often.
EDIT: Nixing this paragraph, asked and answered.
The 5 to 10 comment got me thinking about Cormac McCarthy's Blood Meridian, an admittedly far more grounded piece but ultimately still historical fiction. It struck me that, to the best of my knowledge, nearly every single reference to the story's sole black character by other characters, including the men he rides with, was the n-word. In fact the only time I can think of another descriptor is when one merchant says "people of color," initially before switching to the n-word. It's certainly far less prolific than in Django because he's a secondary character and the book is largely set in Mexico, but as someone without a strong understanding of the word in a historical context, it made me wonder how ubiquitous that term really was. Obviously the n-word is vile by its very nature, but its use as an explicitly derogatory term (as opposed to an insulting descriptor) in this particular text seemed to be confined almost entirely to Indians and Mexicans. Which isn't to say I necessarily buy Tarantino's "that's how they talked" argument, more just food for thought on usage.