The magical, wonderful, oft-abused National Security Letters have been deemed Constitutional, thanks to the vague promise of reviews by courts and government agencies to determine whether or not the normally-indefinite gag orders accompanying them can stay in place.
The USA Freedom Act took away the "indefinite" part of the equation, stipulating that NSL gag orders must be justified by periodic reviews. Unfortunately, "periodic" was left open-ended. The language only specifies "appropriate intervals." It does place the burden on the government to prove that a NSL's gag order is still necessary, but makes no specific demands on how often the government should have to make these assertions.
The FISA DC District Court, however, has specified what a "periodic review" should entail -- at least narrowing down what period "periodic" should mean.
In this order [PDF], a redacted company exercised its USA Freedom Act option to demand a review of gag orders connected to two NSLs it had received. After some in camera presentations to the FISA court, along with some discussion between the NSL recipient and the FBI, it was agreed that the gag orders could stay in place for the time being, but that the FBI should be given the burden of specifying a time frame for periodic reviews, rather than forcing the recipient to file petitions repeatedly until the gag orders were finally determined to be no longer necessary.