For your first question, the longest duration of animal studies that we (DuPont Pioneer) conduct is 90 days. The duration of the 90 day study is not unique to foods from GM crops as the duration is the same as with many other types of products. The 90-day duration was prescribed by international scientific authorities at the Organization for Economic Co-operation and Development (OECD).
In addition to the 90-day studies the registrants of new biotech products conduct, a team of scientists at the University of Nottingham School of Biosciences conducted a review of 12 long-term studies and 12 multi-generational studies of GM foods. These studies ranged up to two years or up to five generations. They concluded there is no evidence of health hazards. Their results are available here.
As to your question about accessing studies, you can find the results from many 90-day studies that have been published in peer reviewed journals at PubMed (http://www.ncbi.nlm.nih.gov/pubmed/) using the search terms “GM food and 90 day rat” and variations thereof. All of the papers discuss the outcome of the individual studies for free in the abstract. Some of the papers are open access which means that the entire paper is also free. Those that are not open access provide a link to contacting the corresponding author which you can do to request a copy.