The length that the kilt should be worn at has evolved over time.The convention these days is that the kilt should 'hit' the middle of the knee. Military kilts have been worn longer in the past but now conform to the same basic rule.
Please note that the length of a kilt is something that CANNOT be altered,as the waist can be. High quality made-to-measure kilts are made so that the 'selvedge edge'(the actual woven edge) of the kilt is at the bottom,and you can even see where the tenterhook marks were. Put very simply,this garment is not hemmed or turned up.