It takes 3.5 minutes…. The time it takes for the soft startup to complete and the amp to switch on.
I used to experience a long warmup for my Audio Research REF 5SE preamp… it took about 12 minutes with signal running through it, and the improvement was substantial. But with the REF 6, that has gone away… sounds great as soon as the 1.5 minutes of soft startup is over and the mute comes off.