I have dry hopped for as long as 4 weeks with no apparent down side. What am I missing here with the short duration?
You'll extract the hop character from hops in just a few days - less than 5 IMO. Past that, some people who keg hop (like me) leave the hops in keg and don't notice a negative to doing it. Others feel that leaving the hops in contact with the beer for too long causes a vegetal character in the beer. Boils down to personal preference.
I don't necessarily believe a long contact time is a surefire recipe for vegetal flavors and I especially do not believe that is the case at serving temperatures in a keg where the breakdown of the vegetal matter in the hops occurs at a much slower pace. There is probably a volume of hop matter in which a given contact period releases enough vegetal matter that it can be tasted but I suspect the volume necessary over any reasonable contact period would exceed any volume one would use in a beer.
The real issues for me are what is the benefit and what is the harm of a longer contact period during dry hopping? Generally the hop oils are procured from the hops in five days or less and I'm not sure what additional benefit is gained from there. On the other hand, the longer the beer sits at warm temperatures the more young flavors in the beer fade. Does the right balance of time favor a longer dry hop? I think most people would say no.