I don't know but I turn mine off after each listening session.  Some leave their Amp on all day whether listening or not but off at night and while out of the house.  Mark.

I have two zen's in two different systems. The first few years with the first zen (going back 15 years now) I would frequently turn the amp off. I experienced a number of tube failures over that time when the amp was switched on cold.

In recent years I turn the amp on and leave it on 24-7 when I'm home. Have not had any tube failures in a long time. I tend to re-tube the amp once a year which is not that expensive for the Zen.

So my experience has been that switching the Zen On & Off frequently could result in one or more tube failures over a 12 month period (especially the rectifier). If I turn it ON and leave it ON the full tube set will go a full year no problem.

It struck me that there is a moral angle to this question.  I did a calculation and given that the Zen amp uses 65 Watts' then leaving it on 24/7 uses a whopping 569 KWh/yr.  If it's used 4 hr/day and turned off the rest of the time it uses 95 KWh/yr.  The difference is about one months electricity use at my house (gas heat).  If other components are also left on with the Zen, then the energy waste really gets big.  

I think I'll turn my equipment off when I'm not using it and buy a few extra tubes.  

I'm not certain, but it seems like I read somewhere on this forum, that once you turn on your amp, you should leave it on for at least 30 min or so before turning it off. That the amp needs at least 20 minutes just to stabilize.
Hopefully Steve or someone can clarify the preferred approach to powering on and off.

The usage of electricity, be it the cost vs. environmental impact, is an interesting one.  Lets assume we are most concerned about cost.  Well, being the one to broach this topic, I will go one step further in asking that we refrain from going philosophical with this one so we can at least limit our variable to just one - the cost.  Someone else can take this to that other realm but I choose to focus on cost.  The average household in America uses 10,812 kWh of electricity per year.  So, by keeping it on 24/7, one uses 569 kWh according to Archie's post.  Now, this is where is gets personal.  Some listen very little, some listen a lot.  And electricity is more expensive in some territories and less expensive in others.  If someone tube rolls and uses rare/unique NOS tubes, the cost of replacement can be considerable.  And how can one value a near impossible NOS tube replacement when said tube(s) no longer exist?  

This being said, I think it may be a good exercise for everyone to calculate the cost per kWh for their city/county and measure thst against their average per day listening. This will help determine if leaving it on all day compared to premature tube replacement makes more sense.  

This of course is assuming that leaving it on all day does in fact lead to longer tube life.  The only other unknown is how much longer life - on average - one can expect to get from leaving it on day and night.  

I tend to believe that the on/off regiment is a much more violent process to the tube than extended "on" time from what I have learned and experienced with incandescent light bulbs.

Not everyone believes in climate change, or that the world is imploding.