There's plenty of evidence out there on how little Americans know. Just look at the proportion of people who believe in Creationism, or doubt global climate change. But this study (abstract here, pdf of full study here), published in Advances in Health Science Education, reviews previous work on the long-term retention of science information. Here's a bit from the abstract:
The results of the review, in the general educational domain as well as in medical education, suggest that approximately two-third to three-fourth of knowledge will be retained after one year, with a further decrease to slightly below fifty percent in the next year.
In other words, knowledge decays. Rapidly. Unless, of course, you use it, but that wasn't really an aim of the studies reviewed here.
A number of the studies reviewed are more educational than aimed at the general public, such as how well students in medical school retain scientific information. Still, there's something to be learned for the rest of us who happen not to be in med school.
The authors do provide near the end some ways in which to enhance science knowledge, or at least its retention, mostly aimed at how to construct a course and curriculum.