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When I use  my AC30 I switch it on and tend to leave it on, putting it on standby when I go out or put the guitar down for a bit.
At the end of the day I turn it completely off. Is this good practice? Does it affect the tube life? Obviously power is being used, but much?

I can't imagine it'll do much damage to the amp .. I suppose valves have a certain lifespan which you're using up but then again they probably have a longer lifespan being left on than constantly turned on and off. There are many old applications in which valves were run constantly for years such as air traffic control systems and so on.

I think valve amps are pretty power hungry compared to most household appliances. There should be wattage ratings on other things in the house that you can compare it to.

Also, personally I wouldn't leave one on if I wasn't in the house, I'd guess there must be a small risk of electrical fire which you'd know about if you were still home but if you were out there's always the chance of coming home to a smoking crater in the ground.

Thanks Mezcalhead. That's pretty much what I was thinking. I thought turning them on and off would cause them more work.
I know in a couple of studio's i've been in the engineer would leave the old valve desk on round the clock. I remember also years back in another studio their AC30 was on a similarworking schedule. Your fire risk point, duly noted. The crater of two evils.

On standby, only the filaments of the tubes are lit.  These are low voltage like 6 to 12 volts (the 6 on a 6L6 means the filament is run on 6 volts, a 12axt filament runs on 12 volts). The high plate voltage I think is off, when on standby. If you turn on your amp on standby first, to give the filaments a chance to warm up and then put it full on, you will maximise life of the tubes.
Leaving it on standby is probably ok, but as Mescalhead says, you dont want a fire hazzard, because the 120 volts ACis still on and is still across the filter capacitors and over time they get leaky and could short out.
The other problem is that heating the tubes causes heat in the chasis, which over time dries out the wiring insulation and it cracks and breaks off.
I would shut the thing off when not using it period.

It is fine.

As hinted uncertainly above, the filaments are the only things that are on in the amp when it is on standby.  EL84s take 6.3V, and in almost every guitar amp I've ever seen 12XXX tubes are wired up to run off of 6V (they can be wired up either way).

Filter capacitors like having current run through them more than they like sitting still.  A capacitor is not going to fail just because current is going through it.  And if it did, the fuse would blow.

The filament in a tube almost never wears out before something else in the tube fails.  The filament is the most durable part of the tube.

I have known people to leave amps on standby for a week or more with no ill effects, HOWEVER, there is something called 'cathode poisoning' which can possibly occur if the tube is left on standby with no plate current for extremely long periods of time.  It used to happen in the old nixie tube clocks.  I have never spoken with anyone that has ever seen it happen in a guitar amp, however.

You are certainly not hurting your amps and I'd say there is a 99% chance you aren't hurting or putting any kind of wear on the tubes, either.