I think this is down to personal choice Wren, I'm having to make the same decision for my Tea Dress. Normally, for Lilya's frocks, I make the lining long enough so that the finished length comes just above the finished length of the dress hem.
For myself, especially for summery things, knee length is long enough, if you think about it, if you wear a slip, that is usually knee length or above. After one particular disaster, I also check the weight of the lining fabric - the finished frock was just too heavy!
Jessie