Theoretically, it's a combination of both methods described by hamsterpower. If you operated the Nest like a regular thermostat: where when you came home you changed the settings, and remembered to set it to "away" before you left the house, it would automatically program itself. That process takes ~2 weeks.
A Nest is unlike a conventional programmable thermostat where you have to think about what time you want it to react. If you leave the Nest in learning mode, it watches when you are home, and when you are not using the motion sensor, and when you adjust the dial. It responds to those adjustments by "learning" how you like things, and also by learning how long it takes to alter the temperature in your house using your existing systems. If I go adjust the Nest to make it two degrees cooler in the house (I'm still running A/C), it will actually tell me how long it will take to get to my new desired temperature setting (Time-to-Temp feature). It also has a toggle that will start cooling or heating early to reach the target temperature by scheduled times (Early On feature).
It has multiple fan settings. One allows you to choose what time span the fan will run to circulate air, and what duration of each our the fan will run (15, 30, 45 minutes, or always on). Unfortunately, that setting is one of the newest features Nest added, and I would have appreciated more flexibility than just one window of time for every day of the week. (not individual days, only all 7 days as a lump).
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