My setup is rarely powered off for more than a few days. That is, the DAC/preamp and digital XO is running 24/7, and so normally the sound has that sense of presence and fullness immediately after powering up the amps (3 of them; one class A and two class D-based) and music server, but after an hour so playing music/movies everything coheres and falls into place a bit better.
8-12 hours after being all-powered up (only on certain weekends, and not with the kWh price of electricity these days) a more pronounced sense of liquidity enters the stage, which must due to the class A amp "settling" more effectively, or so I gather. While a lovely extra addition to the sound this a wee bit more free-flowing sensation is not a "must have" for me to truly enjoy the presentation.
Having had the amps and music server turned off for more than a few days - like a week or two - can lend a stiffness, grayness and slight distance to the sound right after power up, and so the warm up change appears more radical and also takes a little longer. A couple of hours usually suffice here.
Interestingly the most radical boon during warm-up comes from playing my setup relatively loud (i.e.: 85-90dB range, slightly more in peaks) for an hour or so. This is most likely to occur during a listening session with one of my fellow audio compadres or watching a movie at ref. volume, and the effect is quite noticeable. When on my own I sometimes leave the listening room for 15-30 minutes and the music playing in mentioned dB-range, and it has close to the same effect. The sound just "pops" more freely and fuller after such a workout (that is, to my speaker setup it's hardly a "workout," but more like a light evening stroll), and so this is something I desire when listening more attentively. I’m guessing the speakers are to "blame" for this positive effect.