I've had the same signature since I joined. So about four days. xD
I really don't have to change my signature. It's simple, and goes with pretty much any avatar I choose. If I do change it, it's usually only changing the color of the music note on the end, to match my avatar.
Now, I change my avatar regularly. Three times in the last four days, just according to my interests. Mostly, it's my favorite band or song of the moment.