Some of the replies have suggested there is no right answer to this question, I beg to differ. I am not by nature, a tree hugger, but we do have to consider finite limits on the worlds fossil fuels, if not, Global Warming.
I think we have to distinguish units with and without a standby facility, or those that automatically switch to standby when not in use. Units that stay powered on, particularly Power amps and even more so, Power Amps that run warm, usually Class A, are using an appreciable amount of juice. Just how much power does it need to heat up a heavy metal cabinet with components that don't include a heating element.
Something I don't think that has been mentioned, is fire risk. The commonest cause of house fires in the UK, is electrical equipment left on standby, admittedly mainly TVs. I have never heard of a HiFi Rig causing a house fire, but it must have happened.
All in all then, I think kit should be turned off when not in use for a number of hours. I do believe kit needs to warm up to reach optimal sound, but in my experience, this does not usually take more than half an hour.