Here's a good interview by Gaper's Block of Mick Dumke, an investigative journalist with the Chicago Reader, describing his lawsuit against the City of Chicago for their delays and refusals to provide information from Mayor Daley's calendar.
GB: You file a lot of FOIAs against the city. In general, what's your impression of how they usually handle these requests? Do they overzealously redact documents? Do they respond to things in a timely manner?
MD: It varies widely. In some cases, they are very zealous--one could argue overzealous--in redacting information. I've asked for certain police reports and received three sheets of paper with 80 percent of each page blacked out. It's almost funny to look at.
There are certain FOIA officers who are very helpful and give information quickly; who you can sit and talk with and say, "Okay, you don't have time to do it in the five-day period, that's fine--how much time do you need? What's reasonable? If I'm asking for something that's too burdensome, is there something else I can ask for that would give me the same information?" So there are a number of people in the city who are very reasonable and helpful. But, perhaps not surprisingly, I think the closer you get to the mayor, the harder it is to get good, solid, data about what's going on.
Many state FOIA laws allow exclusions for overly burdensome requests, and it's clearly a matter of judgement at many levels what the distinction is between a reasonable request that's fulfilled promptly and an intolerable one that would so disrupt the organization in fulfilling it that it can be rejected as impractical.
Here's a Michigan case, using similar language - in this case "absurdly overbroad" and "intolerable administrative burden" - to define a class of requests which can be rejected out of hand.
Capitol Information Association v Ann Arbor Police Department, 138 Mich App 655; 360 NW2d 262 (1984)
Plaintiff’s request, seeking “all correspondence” between local police department and “all federal law enforcement/investigative” agencies, was “absurdly overboard” and failed to sufficiently identify specific records as required by FOIA, 3(1).
Note that just because a request has a lot of records associated with it, it does not necessarily make it a burdensome request. Just ask those who are getting 26,000 pages of the electronic mail of Gov. Sarah Pailn as governor of Alaska, a request for which the Anchorage Daily News documents the burden of diligence:
Each of those who requested the documents will be required to pay $725.97 in copying fees. They'll also have to pay hundreds of dollars more for the state to ship them what's expected to be about five boxes of copied emails, the boxes weighing about 55 pounds apiece.