Tube amplifier - tube bias and time to warm up

Just speaking generally on push-pull tube amps: 0 mA is perfectly safe, unless the bias reading mechanism is severed (very unlikely) and it’s not actually sitting at 0 mA. In any case, you would see the KT88 plates start to glow orange (usually a large oval patch) and then cherry red as your final warning to quickly SHUT DOWN the amp.

The bias comes up slowly, and you should start to see some reading after a minute or 2. It will increase gradually from there before hitting stability (after ~ 10 minutes as per the manual). But looking for something at 15 - 20 seconds is way way too early.

I’d give it a few minutes, then start adjusting the screw to increase bias until you see a non-zero reading. If you go all the way to the spec’d mA bias early on, it will get too hot later on. So start low and gradually increase. Just keep checking and adjusting (iterate) so it doesn’t run away. You’re a human servo lol. If you’re in trouble you will see the plates glow as your warning. After 10 - 15 minutes do a final adjustment and enjoy.

If it’s using a trimpot you don’t want to overuse that mechanism, so don’t make biasing a daily or even weekly occurrence.

I scanned through the manual and it looks like there's just 1 bias adjustment / reading for all 4 KT88 tubes (cumulative). That probably means you need to stick to tightly matched quads. 

Is matching output tubes necessary if I can adjust bias for each of four tubes ?

Matching is not nearly as critical in that case, but still good to have. A bias reading is measured at just one point (idle), and you want the push / pull tubes to behave symmetrically throughout their operation range. Of course your VAC does it the "right" way - having individually biased tubes. VAC's continuous individual auto-bias since their iQ series is absolutely wonderful - it accounts for tube drift over its lifetime and even during operation. 

I was somewhat surprised that VAC Avatar's manual says nothing about tube matching, even output tubes matching. 

@inna  VAC does recommend sourcing replacement tubes from themselves - which would include the necessary matching and low noise screening for your amp. 

mulveling, this continuous auto biasing, is it done only for sound quality or also for tube longevity ?

VAC's stance is a 3-way benefit: sound quality, tube life, and reliability / fail safes. I can only say that as a user of iQ amps for 4 years, they've been 100% reliable, with no incidents, and never any worry nor need to recheck bias.

If a tube looks like it needs replacement soon, a front panel LED corresponding to that tube is supposed to light up. If it starts to go REALLY bad, a different LED color lights up and the amp shuts off. I haven't had any LEDs light up on me yet. 

I've compared the 200iQ directly to its predecessor Phi 200 (without auto bias) and yeah, the iQ absolutely sounds better. Since this year, I've been running Master 300iQ.