One exception I think (if anyone is still reading up to this point) and that is if your channel changes it's focus audience or genre. Currently my channel art is non de-script since I'm unsure of what I'm focusing on (since deep down I want to do Machinima pieces not LP's but the effort is too much atm) but if I decide to focus more on, say, the Paradox Interactive games then I would brand my channel to reflect that. If within a month I decide that isn't the audience I want to go for after all and want to change to minecraft or CoD then it would be appropriate for re-branding.
I tend to elaborate on all my points but..
TLDR: I think channel art/logo changes should come when you have a reasonable motive for it rather than a strict timeline. (That being said, if you're going for a younger audience then changing it every year would be a way to keep it 'modern'.)