Hair length needed for Extentions?

She probably told you that so that you would have good coverage to hide the bonds and also to blend the hair with the extension hair.  Now with that said, it may not be necessarily true.  I got GL's and my hair on top was only about 4 inches long, but I have lots of layers in and my hair was also very fine, which made it easy for her to blend the too together.  Depending on your hair texture, you may be able to get away with shorter hair on the top layer, but your extesionist needs to know how to blend the hair really well. 

 I'm not an expert, and I know that lots of stylist on this board put GL's in, so maybe they have more answers for you.

I would say in general, the longer that top layer of natural hair is , the more natural looking the extension job will be.

Shorter, thicker hair is much more difficult to get it to blend than finer, thinner hair.

I have done extension jobs with hair that's around 3" on top and they looked ok.  I always tell the client that as her natural hair grows, the easier it will be to blend the extensions.

Ideally, the longer the better, though.