Five years of study and testing.
Mostly done by the industry and by an agency that has repeatedly failed to regulate that industry. The very same organization that is saying that these foods are safe also approved all those fun food additives that are believed to cause cancer and other issues. They're the same folks who were talking about allowing a rebranding of high fructose corn syrup as something else (I forget what, maybe corn sugar) to let the industry hide from consumer backlash over excessive fructose consumption that has been linked to diabetes and heart disease. And the list goes on and on. If we can't trust the FDA—and I maintain that we cannot—then we also can't trust its testing.
And even if we can trust its testing, the harsh reality is that although we know roughly what spliced genes will do in the first generation, under typical circumstances, we can't be certain how these changes could affect naturally suppressed genes over the course of hundreds of generations of breeding, variable environmental conditions, etc. Given enough unrestricted genetic modification, there's a nonzero chance that a previously safe plant or animal could spontaneously stop being so, without warning. Now to be fair, there's a nonzero chance of that happening without genetic modification, but my gut says that the chance is greater in a newly created genetic hybrid than in an organism that evolved over millions of years to be suited to its environment without any of those latent genes getting turned on throughout all of known history.
For those reasons, I feel that people who wish to minimize their exposure to genetically modified foods should have a legal right to know whether a given food product is likely to contain genetically modified foods, even if the additional risk posed by those foods is extremely low, in much the same way that they have the right to know whether pesticide was used, whether the milk was pasteurized, etc. The fact that it is impossible to say with absolute certainty that foods don't contain any genetically modified organisms is mostly irrelevant, because the risk of GMO foods is likely to be extremely small to begin with, so I think the FDA is being disingenuous when they use that excuse to block product labeling. Besides, there's a tiny possibility of pesticide blowing in from the next field and contaminating an organic crop, but the FDA doesn't ban farmers from calling their crops organic. So the FDA is treating this subject differently from other similar issues. That alone is reason to doubt whether they are truly functioning as an independent organization in this regard, or merely bowing to political pressure from big agribusiness.