It varies state by state, but as an example New York State requires 36 hours of continuing ed credits every 3 years. Every credit is 1 hour of classroom/lecture time from an approved sponsor. A lot of people get their CE credits at conventions, which offer easy "one-stop" shopping for many credits over a couple days time. Each state's licensing requirements spell out which subjects are allowed for credit, as well as whether courses can be taken on-line vs. in person (NYS does not allow on-line courses unless they are live-video, i.e. you can interact with the course presenter).