I wear my traditional to the top of my knee, and my Utilikilt to mid-knee. The Utilikilt seems to be designed to avoid flashing people, especially when sitting down. But too long (esp. on a tartan kilt) just looks girly to me IMHO. I have a friend who wears a black workman's UK for carpentry work, he's a manly guy, but he wears it about an inch below the knee. That and sandals makes it look somehow "not right" to me.

Then again, it seems that the highlanders each wore their tartans according to personal preference, before the Victorian rules set in.