If your instrument has been in a stable environment for a while, and you are making a minor adjustment to the truss rods - 1/8 to 1/4 turn - then you should feel/hear most of it immediately.
You can also help it along by applying some gentle pressure with your hands to bend the neck in the direction you are trying to move it. Just hold it in normal playing position, brace one arm against the body, and push or pull the neck a little with your fretting hand.
For small adjustments like this, you may notice a little settling in over night and decide to tweak it a little more, but you'll see most of the effect right away.
Again, that applies if the instrument has been living in the same general heat/humidity environment, you haven't made any major adjustments recently, and you're only changing it slightly. If you switch to a grossly different set of strings or the weather changes, requiring a larger adjustment, then you may need to check and adjust it over the course of a day or two.
I also wouldn't be too paranoid about de-tuning. If you're loosening the truss rods a little, I wouldn't bother at all (unless to make it easier to get to the nuts); if I'm tightening, I just bend the neck a little at the same time I'm turning the nuts.
I'm more likely to de-tune slightly if I want to raise the bridge 1/4 or 1/2 turn, though maybe not to lower it. Common sense works wonders here - if it feels hard to turn the screw or nut, then maybe you should loosen things so you don't risk stripping the threads, but if not then you're not going to hurt anything.
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Looks like Mica and I were writing at the same time, and I could have been working or something...
I'll just reinforce her comment about checking setup in normal playing position. If you measure stuff while it's laying on a table, even gravity pulling down on an unsupported neck is enough to throw off your measurements!
This stuff is neither voodoo, nor rocket science - it just takes a little practice with careful attention.
(Message edited by bob on August 02, 2004)