I've experienced both gradual and "light switch", but most often gradual. The gradual changes have sometimes prevented me from having as much insight into my mood changes as would've been good.

What is often more "light switch" is my change from hypomania to full blown mania. I have a tendency to become hypomanic and then stay hypomanic for quite a while (even up to months), then something triggers it to the next level, and BOOM!. When it does, psychosis can often start quickly, too. Another thing that happens light switch for me is going from euphoric full blown manic to full blown manic with mixed features. That can happen within hours or even minutes, in a couple of cases.