I'm not a field linguist but my hobby overlaps with it, particularly the descriptive part.
At the far ends, with remnant langauges (Virginian Algonquin) and well studied mega-lingua francas like English or French the answer is clear. Given limited time and resources, what constitutes enough dictionary, corpus and other descriptive material to say a language is documented?
One book by a field linguist I read said about six pages of well meticulously glossed text covered most of a languages details. Are there any other rules of thumb for deciding when one has hit the point of diminishing marginal returns for documenting a language?
(I'm not really looking for the answer "the job is never done!", since it isn't really useful)