The first question is similar to asking, "How long is a string?" Arbitrarily long.
I think that "prior art" and "obviousness" claims for certain classes of manipulations should (in a world run by biologists) be useful. For example, if you discover New Gene X and manipulate its DNA to tag its protein product with a fluorescent protein, that's pretty damn obvious because that's what we do with every other protein.
"While the government took the plaintiffs’ side on the issue of isolated DNA, it sided with Myriad on patentability of manipulated DNA."