I've kept the same main ever since I entered second life in May of 2011, but the avatar itself has evolved slowly over the years, adding new hairstyles and outfits and skins, though I noticed as a guy I tended to wear the same outfits and hair for very very long periods of time (which must have bored the hell out of my friends, but that's another story).
I can't remember now which noobie default avi I chose when creating the account, but the same day that I entered I bought the avatar that would stick with me for the duration. Avatar Alfonso by Lilunai Laurasia, which I got for 115 L (yes yes I know I'm cheap) and which unfortunately has been retired and is not available from the Marketplace today.
Nevertheless, by mid-2011 MK was starting to go around in less tattered outfits.
This new hair and shape continued throughout the rest of 2013, though the avatar added a larger wardrobe as the months passed.