Many people like longer workouts because they feel like they "get more out of it" and feel sufficiently spent once they have completed it. Often times, this is the same person who applies equal intensity to all of their CrossFit workouts - no matter the length or style. What that person doesn't realize is that intensity is not a characteristic of a WOD - it is up to the individual. The more effort you put in, the more intense it will be. If you feel, after you finish a 6 minute workout, that you could keep going for another 6, perhaps you needed to push yourself a little harder. Likewise, if you find yourself half way through a workout, suddenly out of breath and focus, having to stop for rest every other rep, you probably need to scale the intensity back. The right intensity for a workout should feel hard to maintain (but doable), a little uncomfortable (but not in pain), and leave you spent (but not dead). Finding the right intensity builds your work capacity efficiently, helping with gains as well as overall fitness.
It is easy to understand this concept with extreme cases, but what about the mid-range workout? What about interval training? Being able to read the whiteboard and correctly gauge the appropriate intensity is a skill that takes time. While you are still learning, here are some general rules - try to aim for an even level of intensity throughout the workout ranging somewhere between 65-95%. If the workout is less than 10 minutes, or if it is intervals involving rest - amp it up. If it is longer than 15 minutes or if involves a technical movement you feel less confident with, step it back. If you aren't sure - ask your coaches, they probably can give you some insight into the intentions of the workout.
Oh yea - if you are coming today... the WOD is a SPRINT!