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When flying IFR under the FAA the departure procedure starts with copying an IFR clearance from ATC.
My question is, how long before departure is it okay to go ahead and copy down the clearance?
Sometimes clearances include void times. The context of my question is for clearances delivered, for example, be clearance delivery at a towered airport. These clearances don't include void times. What't the time limit on these clearances?
You can generally get your clearance 30 minutes prior to the departure time you listed on your flight plan. If you call clearance delivery prior to that they likely won't have it and will tell you to call back 30 minutes prior to departure.
You kind of have two questions in one - When can I call Clearance Delivery to get my clearance? and How long is that clearance good for?
Casey covered the first part - the local controllers will usually have your clearance 30 minutes before your scheduled departure.
Sometimes you can get this information a little earlier yourself - the new Flight Service tools like 1800WXBRIEF.com can give you expected clearance and flight plan routing information based on what you filed, if the ATC computer already knows what it's going to do with your flight. Here's a few words about it from the Foreflight folks, and you can do the same thing yourself with the Lockheed-Martin tools with a few more manual steps.
You still need to talk to the controllers but if you know what they're going to say it makes that conversation faster!
The second part can go one of two ways: The point of the "void time" is that the ATC system is reserving a spot in the system for you up until that specific time. After that ATC is going to let other aircraft use the airspace they were holding for you (they may allow someone to make an instrument approach to the airport you're leaving from, for example).
This is used a lot for non-towered fields where you have no local controller to talk to.
Clearances from Clearance Delivery at a towered field can include a void time, but they generally don't because they assume you're in the plane and ready to go at or around the time you call in, and you have local controllers to talk to if you have a problem during the run-up or something which delays your departure. Basically the clearance is good until someone tells you otherwise in that situation.
The tool that gets used at towered fields more often is Hold for release: You've been issued a clearance, but you can't execute it until the tower has coordinated you a spot in the airspace. This is generally more efficient than reserving airspace with a clearance and void time when there are controllers to talk to, and if everything is working well your release should be ready right around the time you're ready to take the runway for departure.
As casey and voretaq correctly indicated, your flight progress strip containing your route and other information (most pertinently your computer-assigned transponder code) does not become available to the controllers in the tower until 30 minutes prior to your filed "proposed departure" time. You can call sooner than that, but say your callsign very clearly and not super fast—and you might add in something along the lines of "we're a little early." If the controller has a moment, they can edit your P-time and force your strip to print.
The "time limit" on these clearances is set by the overlying ARTCC. Beacon codes are a limited resource and we don't want flight plans holding on to them longer than necessary, so there is a "flight plan drop interval" set for all pending flight plans in that ARTCC's computer. Generally this drop interval is two hours after your P-time—in other words, the window where you can call to copy your clearance is 30 minutes prior to the P-time until 120 minutes after it. During times of long delays in the NAS that drop interval may be increased to three or four hours.
My personal technique is to ensure that any clearance which has been issued to a pilot will not drop out. If the strip marking shows that a clearance has been read, I will keep pushing back the P-time so you don't have to re-file when you're ready. But this is not a guaranteed service, so if you're running late it would behoove you to call again and request that we adjust the time for you.
