I doubt any body can possibly last longer then 1 month in warm weather. the whole body wouldn't even need to rot. only enough so that the cadaver can no longer support itself or use its limbs with any force or the nerves to degrade to the point where they can no longer carry impulses from the infected brain. In cold weather I can possibly see a couple months but given how warm summers can be plus the pretty much constant humidity in eastern north america it adds up to produce a pretty short lifespan. another factor are bugs. assuming the virus is endemic to humans which always appears to be the case i can guarentee that a number of flies , beetles, and other insect species will place they're larvae on the zombie for food which will cause the body to decay even faster. I do not forsee the zombies sticking around for long in the summer. While you do have a point in the infection rate as potential hosts become rarer so will zombies. After the initial panic however the remaining population will wise up and begin entrenchment and hiding the number of deaths will fall dramatically. As victims become less common and more resourceful the zombies will likely sharply decline within months. and the whole crisis will be almost over within a year leaving the clean up to the survivors.
