My Mum has had short hair all of her life, she certainly has never had it past her shoulders at any rate. But I have *finally* managed to convince her to let it grow, to see how long she can get it. But I think she gets very discouraged because her hair doesn't grow very quickly, and her own Mother always said that she'd never be able to grow her hair very long, simply because as a child it never grew very long (but then again perhaps my Nan cut it short all the time, I'm not sure).

But since my Mum's been attempting to grow her hair, I have heard that the eventual length of your hair is also determined by genetics. Is this true? Or can long hair be attained by anyone, including my Mum?

Thanks 

A hair's terminal length is indeed determined by genetics, as is everything else. But most hairs get cut or broken off way before they reach their terminal length.