Last year I changed the look monthly. Or maybe that was earlier this year. Last year might have been seasonally. I like to take photos and have them as my background. Currently I am using a theme from http://www.thecutestblogontheblock.com/ I'm more likely to change the look with various hacks and widgets.
While I am almost always adding new buttons or images that suit me or help with networking, as far as changing the layout (or banner in my case), only once a season (or holiday) for me. It takes a good amount of time to play with a blog, and though I want the look to suit my personality, I don't have the time to do it every month. In fact, it's about time for me to change my banner, now that Christmas is over. :) Might as well do it while we're still on winter break and I have the time!
I used to update the look frequently, but I won a blog makeover over the holidays, so I'm hoping to keep my design put for a long time. As far as adding things on the sidebar....I'll be doing that all the time.
As far as others who have asked if it makes sense to pay for a pro layout/design....good designs really do have an impact on the way I feel about a blog, and whether or not I'll be likely to come back. It's strange, but it really makes a difference.
I've had the same layout for my main blog for a while now. It's come to the point where if people see my blog header, they automatically think of me. I don't think I could change it even if I wanted to.
Now, my review blog is a different story. I'm constantly messing with it and having people re-design it. I don't even want to think about how much money I've already spent changing the look. In fact, I have someone re-designing it at this very minute! LOL!