Whenever I keep a theme for a long time, that usually means I like it. I like to try different things on, and if I don't like something I'll chuck it.
As far as themes go, I really liked my Layne Staley theme, which is why I made an ode to it in my siggy. I really hate recycling themes, though, unless they have nostalgic value. The one avatar I'll go back to every so often is my old avatar from Bulbagarden back in 2009, when I was known as RockSmashGuy. It's so pretty and unique, and... so me. Nothing comes closer than my original~