I've changed mine a couple of times here so far. How long I keep an avatar or sig depends upon how happy I am with it, because yes, I think they reflect very highly upon the person using them. Usually I keep an avatar/sig set for 5-6 months, since I don't really like to change them very often. People who contantly change their avatar/sig come across as being abit indecisive and restless.
This was my first one and I used this for a long time.. about a year. It was just a random image I decided to have 'as me'. But since then, my sigs and avatars go with the anime or mangas i'm most taken with.
... then Bubblegum Crisis.
But for now, i'll stay with Sang and PHD. I wasn't really content with my others, which is why I didn't keep them for very long. But for now, i'm most happy with what i've got.