What Tiny said. If you're going to idle it to full temp, 20 - 30 min ought to do it, especially if the thermostat is working correctly. The thing is, not only does the coolant temp have to come up, everything else has to follow it to operating temperature, and be levelled off for a bit before you can get an accurate adjustment.
Adjusting to spec (by the book) is done with the engine shut down, immediately after the warmup so everything hasn't cooled yet. You can do it by ear with the engine running, but, unless you have plenty of experience you won't get the clearances as accurate as you would by the book, with a gauge. If it is your practice to idle the truck around a flat town, you can set it tighter, but if the truck works hard, set it looser(e.g., exhaust .015 rather than ,013)so that the engine can be run hotter without hanging up the valves. Too loose is noisier, but better than too tight. 4onthefloor is dead right about that one.
