For almost every theme I've ever worn, I have made them myself...and I mean completely. I draw the illustration and make the graphics, so you can imagine I take a great deal of pride in the things I do. That said, I wouldn't change often at all, and would really only change when I make something that can be deemed good enough to me for a change.
I'm...one of those people who generally don't believe in changing themes very often at all, heck, there are numerous times during my PC life where I've kept my themes for 9 or more months, maybe more. I just feel like my themes help show off my identity; who I am, and if I make something that I feel can do a better job of it, then I'd change. But otherwise...you can see me keeping the same theme for a very long time.
As for my profile...well, I change the layout of it just as rarely, perhaps once every year? o_O