How long does it take your tube amp to warm up?

My tube amplifier has about 12 tubes and takes a couple hours for it to open up and sound really good and comfortable to listen to. Always wondered why it takes so long for a tube amplifier to warm up and what’s happening during this phase.

If I left it on for six or seven hours would it sound even better? I normally don’t leave it on for more than four or five hours.

How long does it take your amplifiers to warm up?