CMcG, I too wore the kilt a handful of times years earlier before learning how to do it right. I visited a gentleman in Edinburgh who was a friend of a friend and his advice was still good. He said to wear it with a "jumper" and make it a casual garment like any other casual garment, rather than putting on a costume. So the first two times I wore a kilt I wore it with an Izod Chemise Lacoste tennis shirt. The first was to a drinking golf tournament and the second was to a horse race. Ten years passed before I put on a kilt again and the next occasion was a little dressier. And I rose to the occasion, more or less. While I am as staunchly traditional as you'd want to be, I still wear my kilt on occasion with a sweater or a knit shirt, just as I do khakis or grey flannels.

As for how long, I think Jock is right as usual, the tongue in cheek voice of authority usually wins attention, if not respect. Just remember, the only man who has worn a kilt his entire life is a dead man. The rest of us may have worn it all our lives so far, but that is not the same thing.