Tube Life: 6SL7; 6SN7, 12AX7, 12AT7, 6DJ8 et al

Turn it off when not in use.

Don't turn it off when you take a 'phone call.
Kill it all when you have left for better thngs...

Turn it off only when you are not going to listen for more than a few hours. Tubes need time to warm up to sound their best, normally 30 min or more. You don't want to waste too much tube life just for warm up.

I have a similar concern. I use my Aesthetix pre-amp standby function when I am not listen to music. In fact, I hardly turn off the unit completely. I leave it "Standby" day and night; unless I am out of town for days. I have used the tubes (6922 and 12AX7W) in this manner for over 3 years; they sound fine (I think?).

Now, what's your recommendation for the proper use if the unit has the "standby" function? And what's the life expectancy on those tubes with proper use?

If your gear is equipped with a standby switch, no power is being applied to the tubes. In most cases, it's purpose is to allow you to turn the unit on via the remote. Standby is much different than leaving the gear on "hot" with "tubes lit" as was the initial question.

I've always turned me tube gear off when not listening to them.
I usually turn everything on and let it warm up for half an hour or more before a listening session.
I've had tube gear for so long, that out of habit, I plan when I'm going to do some serious listening, usually for a couple of hours or more, then shut everything down when I'm done.
That way I don't have to worry about constantly turning the system on and off.
Some tube preamps, like the older Audible Illusions Modulus, would keep the heaters on the tubes constantly on, even with the power turned off.
When in standby mode, peek into the preamp and see if the tubes are still glowing.
If they are, it's better to completely shut it down.
With the cost of tubes going through the roof lately, you want to get the most mileage out of them as possible.
A bigger problem with tube life is that some manufacturers run the tubes really hard, which drastically shortens their life.
CJ was infamous for this on their preamps, and would burn out preamp tubes at an alarming rate.
Some tubes are also more fragile than others, and have a much shorter life.
I've never had good tube life out of Telefunken 6DJ8's.
My Klimo Merlin would kill a pair of Telefunkens in a month.
Yet, I ran a set of Tungsram 6DJ8's in this same preamp for more than five years!

Ontjesr,
The standby switch on many tube preamps only cut off the power to plate but not to filament. Some may run the filament at a reduced voltage. Once the plate current is cut off, the tube is not doing work but the filament is still lit and maintaining optimum working temperature. It is not just for the remote.