A month should be ideal for a round trip to allow you enough time to stop and enjoy the places and soak in the scenery.

The speed record is hold by John Ryan (rip) who  rode his motorcycle from Prudhoe Bay Alaska to Key West Florida in 86 hours 31 minutes. But that is John Ryan and he is the hero of long distance motorcycling. Many people did a round trip in two weeks. But this might put undue stress on the motorcycle and your body. It will cause you to skip many great locations and views. In a two week trip, you may find yourself forced to eat miles and not really enjoying the experience, wishing it to end asap, that may very well defeat the purpose:having good time.