Look up different publishers. 50-70K for Mills & Boon-type length - goodness knows how many for War & Peace! Around 120/140K is about average, I believe.
Meant to add that I stick to short stories and they can go from vignettes (around 750) to about 3,000 - but well-know authors often do more. I've only published 2 short stories, both were slightly under 3,000.
I think that internet publishing is loosening things up - I don't know how to do it, alas, so am stuck with the stamps and return envelopes!