Impossible to give a meaningful answer. I can camp and cook on long tours with that much or less capacity. Someone else might not have enough room for their clothes. It depends on what you want/need to have along. I'd suggest assembling the stuff you will take and do a test packing. When it comes down to it, staying in motels and eating in restaurants you really aren't going to get into too much trouble by making bad packing decisions. You'd survive with nothing but a credit card and taking too much would just mean being unnecessarily loaded down. Neither would be the end of the world.

Bottom line... For me that would be very generous amount of packing space for the trip. It might be nice if you plan to want a lot of off bike clothes or something. Otherwise maybe it is overkill. It depends on how you pack though..