YMMV, but if I get tired of my own voice I find I'm usually doing something wrong. I re-wrote my first novel (http://www.amazon.com/Darwins-Theorem-TJ-Radcliffe-ebook/dp/...) dozens of times, and likely read it over a hundred, and I could tell what passages needed work by how I reacted to it when re-reading. If it felt stale, if my eyes glazed over, if I wanted to skim ahead to find where something interesting happened, then the reader would likely to do the same.
One my first readers commented that she found the final version of the book vastly more readable than the earliest one she read, precisely because the voice was livelier and more varied. In the earlier draft she found it "sounded" too much like me all the time, and while my natural voice is not totally boring, too much of any one thing gets dull after a while. Readers invest a lot in our writing, and deserve to rewarded for it in as many ways as possible, from the ideas and characters we show them to the pleasure of the words flowing through their brain.
In non-fiction I've not found this to be such a big problem, although I'm looking less for artistic effect there and much more for clarity, although I don't think I've written anything over 100K words in non-fiction (plenty of things in the mid-10K range, though.)
One my first readers commented that she found the final version of the book vastly more readable than the earliest one she read, precisely because the voice was livelier and more varied. In the earlier draft she found it "sounded" too much like me all the time, and while my natural voice is not totally boring, too much of any one thing gets dull after a while. Readers invest a lot in our writing, and deserve to rewarded for it in as many ways as possible, from the ideas and characters we show them to the pleasure of the words flowing through their brain.
In non-fiction I've not found this to be such a big problem, although I'm looking less for artistic effect there and much more for clarity, although I don't think I've written anything over 100K words in non-fiction (plenty of things in the mid-10K range, though.)