This website gives you statistics for the expected amount of shared DNA for various relationships.
http://www.isogg.org/wiki/Autosomal_DNA_statisticsIf she is your full aunt (i.e. full sibling to a parent) then you would expect to share 25% of your DNA (or about 1700 cMs total). If she is your half-aunt, (i.e. half sibling to your parent) then you would expect to share about 12.5% (or 850 cMs.) Note that these are averages -- it can and has been different for some people since DNA inheritance is random.
Also, here is a good beginners guide to understanding genetic genealogy:
https://sites.google.com/site/wheatonsurname/beginners-guide...You can also read blogs by various people that are basically all about genetic genealogy. The ones I read (and re-read because I sometimes don't understand things until much later as I learn more) are:
The Genetic Genealogist
Your Genetic Genealogist
DNAeXplained
Kitty Cooper's blog
Hope this helps!
Elizabeth